Fixes the consistency of several arguments in operator= and assign
and the constructor:
basic_regex from unnamed, that, e -> e
basic_string from p -> s
const charT* from ptr, p -> p
flag_type from unnamed -> f
initializer_list<charT> from unnamed -> il

These changes are in both [re.regex] and [re.regex.assign].

Moves the [re.regex.assign] LaTex reference in [re.regex] before the
operator= overloads to match the [re.regex.assign] section.

There is one more inconsistency [re.regex] declares:
basic_regex& assign(const charT* p, size_t len, flag_type f);

[re.regex.assign] declares:
basic_regex& assign(const charT* p, size_t len,
  flag_type f = regex_constants::ECMAScript);

Since this is not an editorial issue I'll file an LWG issue.