Here's some suggested changes to add some more details and do some cleanup  
in '3.6. Tasks':

 * '3.6. Tasks' and '3.6.1. Promoting a Function to a Task' repeat
   the same example verbatim. '3.6. Tasks' could be shortened to just the
   introductory paragraph, and could also be simplified a bit. For example,
   addtask forces task names to start with "do_", so calling it a convention
   might give the wrong idea.

   Here's a suggested new version of '3.6. Tasks':

     Tasks are BitBake execution units that make up the steps that BitBake can
     run for a given recipe. Tasks are only supported in recipes and classes
     (that is, in .bb files and files included or inherited from .bb files).
     Tasks have names that start with "do_".

 * '3.6.1. Promoting a Function to a Task' could then be rewritten as follows:

     Tasks are either
     [shell functions](https://www.yoctoproject.org/docs/2.2/bitbake-user-manual/bitbake-user-manual.html#shell-functions)
     or
     [BitBake-style Python functions](https://www.yoctoproject.org/docs/2.2/bitbake-user-manual/bitbake-user-manual.html#bitbake-style-python-functions)
     that have been promoted to tasks by using the 'addtask' command. The
     'addtask' command can also optionally describe dependencies between the
     task and other tasks. Here is an example of a task being defined, along
     with some dependencies being declared:

       python do_printdate () {
           import time
           print time.strftime('%Y%m%d', time.gmtime())
       }
       addtask printdate after do_fetch before do_build

     The first argument to 'addtask' is the name of the function to promote to
     a task. If the name does not start with "do_", "do_" is added implicitly,
     which enforces the convention that all task names start with "do_".

     In the example above, the 'do_printdate' task becomes a dependency of the
     'do_build' task, which is the default task (the task run by 'bitbake'
     command unless another task is specified explicitly). In addition, the
     'do_printdate' task becomes dependent upon the 'do_fetch' task. Running
     the 'do_build' task will result in the 'do_printdate' task running first.

     Note {
     If you try out the example above, you might see 'do_printdate' only being
     run the first time you build the recipe with 'bitbake'. This is because
     BitBake considers the task "up-to-date" after that. If you want to force
     the task to always be rerun for experimentation purposes, you can make
     BitBake always consider it "out-of-date" by using the
     [[nostamp]](https://www.yoctoproject.org/docs/2.2/bitbake-user-manual/bitbake-user-manual.html#variable-flags)
     variable flag, as follows:

       do_printdate[nostamp] = "1"

     Another option is to explicitly run the task with the '-f' flag, as
     follows:

       $ bitbake <recipe> -c printdate -f

     When manually selecting a task to run with 'bitbake <recipe> -c <task>',
     the "do_" prefix on the task can also be omitted.
     }

     You might wonder what the practical effect of using 'addtask' without
     specifying any dependencies is, like in the following example:

       addtask printdate

     In this case, assuming dependencies have not been added through some other
     means, the only way to run the task is by explicitly selecting it with
     'bitbake <recipe> -c printdate'. The 'do_listtasks' task can be run to
     list all the tasks defined in a recipe, using
     'bitbake <recipe> -c listtasks'.

     For more information on task dependencies, see the
     [Dependencies](https://www.yoctoproject.org/docs/2.2/bitbake-user-manual/bitbake-user-manual.html#dependencies)
     section.

     See the
     [Variable Flags](https://www.yoctoproject.org/docs/2.2/bitbake-user-manual/bitbake-user-manual.html#variable-flags)
     section for variable flags that can be used with tasks.

 * As an unrelated change, the [noexec] and [nostamp] description in
   '3.7. Variable Flags' could be clarified a bit, and a warning could be
   added:

     [noexec]: When set to "1", marks the task as being empty, with no execution
               required. <same as before...>
  
     [nostamp]: When set to "1", tells BitBake <same as before...>
  
                Caution {
                Any task that depends (possibly indirectly) on a [nostamp] task
                will always be executed as well. This can cause unnecessary
                rebuilding if you are not careful.
                }


Cheers,
Ulf