Restructure re.sub docs, clarify aspects of repl notation

- `flags` are only relevant when `pattern` is a string (followup to python#119960).
- Moved simplest "beans and spam" example from under "repl is a function"
  (which it wasn't!) close to start, extended to demonstrate
  both string & re.compile flags usage, and `\1` templating.
- Discuss all how-we-match parameters before what-we-do-with-matches.
  TODO: Is important info close enough to start?

- Explain callback before backslash notation because it's shorter but also
  to promote it. IMHO, people fear it as a "last-resort escape hatch"
  while it's actually *simpler* than backslashes (python#128138 is one example).
  TODO: Will this order make sense after python#135992 ?

- Consolidated `repl` notation from two far-away paragraphs to one place.
  - Starting from `\1` and `\g` which are the whole purpose of dealing with backslashes!
  - Briefly mention `\octal` wart, 99 limit and `\g<100>` avoiding them.
  - Draw attention to `\\` for getting a literal backslash.
  - Clarify that *most* escapes are supported but `\x\u\U\N` aren't.
  - Move "Unknown escapes of ASCII letters" *after* listing all the known ones.
  - Added a note promoting raw string notation for `repl` too.

Author: cben

Doc/library/re.rst

@@ -1089,7 +1089,7 @@ Functions
 
    *repl* can be a string template or a function:
 
-   * If it is a function, it is called for every non-overlapping occurrence of
+   * If it is callable, it is called for every non-overlapping occurrence of
      *pattern*.  The function takes a single :class:`~re.Match` argument, and
      returns the replacement string.  For example::