Clarify map definition.

Requested by @wfspotz.
Fix errors in dimdata summary table.
Tweak language around padding.

Add "boundary padding" and "communication padding" to Definitions section, though the definitions are currently TODO.
Make a couple of minor formatting tweaks.
Merge branch 'merge-and-remove-components' of https://github.com/enthought/distributed-array-protocol into merge-and-remove-components