Due to a mistake in the grammar, a dollar character implicitly escapes a second
dollar character that immediately follows, so that it cannot start an
interpolation. Unfortunately, this behaviour has since come to be relied upon,
so it cannot be fixed. Furthermore, the documentation on regular strings did
not mention this behaviour at all, while in the case of indented strings it was
rather implicit.
Mention it explicitly in both cases, and describe how an interpolation can
follow a dollar character (namely, by escaping that). Since we have to touch
that section anyway, state that any character (other than n, r, and t; but
notably including `$` even if not succeeded by `{`) can be escaped using a
backslash in regular strings.
Change-Id: I7e5d68a9a4130eec98ce8218b485168f4b31a677
beb231784e