When scanning CMake module files for .rst comments, recognize
bracket comments starting in ".rst:" too.  For example:

 #[[.rst:

Include the bracket comment content terminated by the closing bracket.
Exclude the line containing the bracket if it starts in "#".

Teach the CMakeLib.testRST test to cover multiple bracket lengths
and ending brackets on lines with and without "#".

Update the cmake-developer.7 manual to document the bracket-comment
syntax for .rst documentation.