Specify module index descriptions using an attribute rather than a comment

[bradcray]

Currently, the descriptions of the modules that appear in the module index document are generated by convention using the first line of the multi-line comment that precedes the module declaration. As noted in #25667, this is sufficiently subtle and fragile that we've sometimes ended up picking up comments that were not designed or intended for that purpose, such as the file's copyright, the first line of a multi-line comment that wasn't intended for the module index, etc.

This issue requests that, now that we have attributes, we update the process of generating the module index to use a special attribute like:

@chpldoc.modIdxDesc("This is the description for my module")
module MyModule { … }

(where I have no attachment to this particular spelling of the attribute's name.