curl: --help [option] displays documentation for given cmdline option

Since the documentation text blob might be gzipped, it needs to search
for what to output in a streaming manner. It then first searches for
"\nALL OPTIONS".

Then, it looks for the start to display at "\n    -[option]" and stops
again at "\n    -". Except for the last option in the man page, which
ends at "\nFILES" - the subtitle for the section following all options
in the manpage.

Test 1707 to 1710 verify

Closes #13997

docs/TODO

@@ -148,7 +148,6 @@
  18. Command line tool
  18.1 sync
  18.2 glob posts
- 18.3 -h option
  18.4 --proxycommand
  18.5 UTF-8 filenames in Content-Disposition
  18.6 Option to make -Z merge lined based outputs on stdout
@@ -1037,12 +1036,6 @@
  Globbing support for -d and -F, as in 'curl -d "name=foo[0-9]" URL'.
  This is easily scripted though.
 
-18.3 -h option
-
- Support "curl -h --insecure" etc to output the manpage section for the
- --insecure command line option in the terminal. Should be possible to work
- with either long or short versions of command line options.
-
 18.4 --proxycommand
 
  Allow the user to make curl run a command and use its stdio to make requests

docs/cmdline-opts/_OPTIONS.md

@@ -24,3 +24,5 @@ clean option state, except for the options that are global. Global options
 retain their values and meaning even after --next.
 
 The following options are global: `%GLOBALS`.
+
+# ALL OPTIONS

docs/cmdline-opts/help.md

@@ -2,7 +2,7 @@
 c: Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.
 SPDX-License-Identifier: curl
 Long: help
-Arg: <category>
+Arg: <subject>
 Short: h
 Help: Get help for commands
 Category: important curl
@@ -12,15 +12,28 @@ See-also:
   - verbose
 Example:
   - --help all
+  - --help --insecure
+  - --help -f
 ---
 
 # `--help`
 
-Usage help. List all curl command line options within the given **category**.
+Usage help. Provide help for the subject given as an optional argument.
 
 If no argument is provided, curl displays the most important command line
 arguments.
 
-For category **all**, curl displays help for all options.
+The argument can either be a **category** or a **command line option**. When a
+category is provided, curl shows all command line options within the given
+category. Specify category `all` to list all available options.
 
-If **category** is specified, curl displays all available help categories.
+If `category` is specified, curl displays all available help categories.
+
+If the provided subject is instead an existing command line option, specified
+either in its short form with a single dash and a single letter, or in the
+long form with two dashes and a longer name, curl displays a help text for
+that option in the terminal.
+
+The help output is extensive for some options.
+
+If the provided command line option is not known, curl says so.