Add a new "document selected symbols" feature

[palacaze]

This would allow the user to add doxygen tags to parts of a file in one go.

I foresee heavy use of such feature in the context of files where a few symbols need documentation but not quite all of them, or to specifically avoid a few symbols. A typical example would be commenting all the entries of an enum (and even better, align the short doc strings but that is for another issue).