Fix up use of API doc references in the changelog

Removed several instances where single-backticks were used around words that do not correspond to any references in the docs.  But also converted those that *were* references to the API docs to double-backticks, for two reasons:

 1) Use of double vs single backticks have not been used consistently
    in the changelog to begin with; for the sake of consistency
    double-backticks is by far the easier default to fall back on.

 2) This is intended to be a long-term document, and many API references
    will become invalid over its history.  Several are already invalid.
    While it is highly useful to link to *valid* API docs from a
    long-term maintenance view it is easier not to.

Also a few small typo and wording fixes here and there.

It's a shame to give up the API references that *did* work, and if anyone feels strongly about it I can go back at least through those that are still relevant and update them. 

In the future I'm thinking of adopting a slightly different changelog format, where instead we only use CHANGES.rst for the "current" release series, and maybe a HISTORY.rst for older releases.  That might also alleviate this issue, though I haven't given it full consideration yet.