Quote less in manuals

Quote less in manuals

The manuals often used quotes ``...'' when it is better to use @dfn or
@code or capitalized words or no quoting at all.  For example, there is
no need for the `` and '' in “if a variable has one effect for
@code{nil} values and another effect for ``non-@code{nil}'' values”.
Reword the Emacs, Lisp intro, and Lisp reference manuals to eliminate
unnecessary quoting like this, and to use @dfn etc. instead when called
for (Bug#21472).