Make Block (and other) distributions lean on chpldoc more (?)

[bradcray]

Today, the chpldoc documentation for modules like BlockDist.chpl are a monolithic chunk of text at the outset of the module. I believe that, historically, this was because these modules only really exposed one type, and so most of what we needed to do was explain what that type did and how the distribution worked as a whole. However, over time, we've introduced a number of factory methods and user-facing initializers that could be documented using chpldoc but, today, are done by manually putting chpldoc-like rst into the monolithic header. Beyond the maintenance burden of keeping those doc-oriented prototypes matched to the code, an impact of this that @jabraham17 caught is that we can't have @deprecated messages on such routines contain hyperlinks in the docs to their replacements. Another is that when we rename things like Block to blockDist, we don't have chpldoc-umentation corresponding to those symbols to update.

This issue proposes that we refactor the BlockDist documentation so that it can lean more on chpldoc for things that are meant to be user-facing and get out of the practice of manually mimicking it.

[bradcray]

Apologies for accidentally at-tagging user deprecated. :(