Modules docs: move rst docs into bracket comments

CMake >= 3.0 supports bracket comments, and the reStructuredText integration code in docs/sphinx/ext/ecm.py already supports extracting the docs from a bracket comment instead. Editing documentation without leading line comment markers is more simple, e,g. when reflowing text over lines.

With ECM meanwhile requiring CMake 3.5 now it is possible to switch (and thus follow also the approach used by cmake itself).

If there are no objections (or enough +1) until WE April 24th/25th I will move on and merge.

And with the diff being not that easy to read, what basically is done is thie:

#.rst:
# bar
# foo

# license

becomes

# license

#[=======================================================================[.rst:
bar
foo
#]=======================================================================]