Add doc of @filename in intro_patterns.rst

I noticed that the @filename notation for the --limit argument was not
conveniently described.

This patch adds a simple example to the intro_patterns.rst file.
Fix several it's/its errors in the documentation.
Add readlink filter doc

As requested at request pull #9213, I'm proposing updathing this doc. Do you have man pages where I can update this too ?

Thanks
Add one line example for looking up through etcd
Make dynamic inventory return byte str, not unicode
Initial work to refactor Task
Merge pull request #9252 from kim0/patch-1

Add one line example for looking up through etcd
Merge pull request #9250 from i5513/patch-2

Add readlink filter doc
Merge pull request #9239 from direvus/grammar-its

Fix several it's/its errors in the documentation.
Merge pull request #9237 from jkleckner/add-limit-from-file-doc

Add doc of @filename in intro_patterns.rst
An attempt at modularization prior to moving things towards BaseObject and considering Attributes.
Merge branch 'refactor_pass' into devel
Fix docstring.
Refresh submodule pointers.
Submodule update
Fix order of resolving dependent role variables from vars files

Also adds a new class to the test_var_precedence test to check for
this type of error in the future.

Fixes #9178
This is an exceedingly rough sketch of what attributes might look like - metaclass implementations NOT complete.
Add attribute starter notes
WIP on refactoring changes
Update submodule reference