api-parse-options.adoc: document hidden and OPT_*_F option macros

api-parse-options.adoc: document hidden and OPT_*_F option macros

In "Documentation/technical/api-parse-options.adoc", the list of option
macros does not mention the `OPT_*_F()` macro variants that take a
trailing `flags` argument, nor the `OPT_HIDDEN_GROUP()` and
`OPT_HIDDEN_BOOL()` convenience macros.

Now that a previous commit documents the per-option flags, let's
document these macros too:

  - Add a paragraph explaining the `OPT_*_F` convention and how it
    relates to the per-option flags.

  - Document `OPT_HIDDEN_GROUP()`, introduced in a previous commit,
    right after `OPT_GROUP()`.

  - Document `OPT_HIDDEN_BOOL()` right after `OPT_BOOL()`.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>