[Chapel Merge] Further improvements to the spec, especially w.r.t

Branch: refs/heads/master
Revision: 790551a
Author: lydia-duncan
Log Message:

Merge pull request #16434 from lydia-duncan/moreDocsRearrangement

Further improvements to the spec, especially w.r.t. use and import statements
[reviewed by @mppf]

This PR splits some of the larger portion of the Using Modules section into relevant subsections.
It also moves some duplicated wording from Using Modules and Importing Modules into a shared
place, and adds some new descriptions of conflicts both there and in the variables chapter.
Finally, it replaces some lingering “references” with “accesses” in the Qualified Access section
(since references are a different concept in Chapel).

Resolves Cray/chapel-private#1288

Double checked the built docs, and passed a fresh checkout of all the spec tests

Modified Files:
M doc/rst/language/spec/modules.rst
M doc/rst/language/spec/variables.rst

Compare: https://github.com/chapel-lang/chapel/compare/72cefe145cd2...790551a5dfc4