Remove almost all unpaired backticks in docstrings (#119231)

As reported in #117847 and #115366, an unpaired backtick in a docstring
tends to confuse e.g. Sphinx running on subclasses of standard library
objects, and the typographic style of using a backtick as an opening
quote is no longer in favor. Convert almost all uses of the form

    The variable `foo' should do xyz

to

    The variable 'foo' should do xyz

and also fix up miscellaneous other unpaired backticks (extraneous /
missing characters).

No functional change is intended here other than in human-readable
docstrings.