Add cross-references for "permissible types"

[t3nsor]

The term "permissible types" was added by P1787R6 "Declarations and where to find them", which states that its wording is relative to N4861. In that wording, the uses of "permissible types" in [over.match.copy], [over.match.conv], and [over.match.ref] are referencing a term that was defined in their parent section, [over.match.funcs].

However, the hanging paragraphs at the beginning of [over.match.funcs] were soon thereafter moved into subsection [over.match.funcs.general], which is a sibling to the subsections that use the term "permissible types", therefore cross-references to [over.match.funcs.general] should be inserted in the sections that use the term.