Fixed #20104 -- Changed VersionDirective in order to avoid ambiguity.

As explained in ticket #20104, the use of versionchanged/versionadded
was confusing.

To solve this ambiguity these directives no longer accept a second
argument but now they only receive the version number (1st arg) and then
a content with the proper comment.

This includes:
	* Modified logic to avoid ambiguity.
	* Modified all the occurrences of those directives to adapt to new form.
	* Unused imports removed.
        * Updated writing-documentation.txt to reflect the changes made.

This PR refs #20104