Rework Completions and Externs chapters

[NotTheDr01ds]

Started out implementing #1044 and #788.

• Moved Completions chapter ahead of Externs. This reduces duplication since Externs can take advantage of Completions.
• Extensive wording cleanup
• Document that completers must return a list
• Document that the default completer is for files/directories
• Document that the default completer can be suppressed using a command that returns an empty list
• Mention the menu-indicator and how to change it
• Clarify that changes to completers require reparsing the command-to-be-completed
• Added documentation on the $position argument (should close Document arguments to autocomplete commands #788)
• Updated example to use cleaner match syntax
• Use Tab in place of tab (at some point I'd like to update the style on this)
• The External chapter used git push as the example, and said it was in the default config (which it isn't, at least not any longer) -- Changed example to be a simple ssh example that demonstrates just the key points, much more succinctly.
• Changed glob to path in the second example (should close There seems not to be any document about path as a type. #1044)
• Changed warning for # description to just a tip
• Added limitation note regarding caret sigil
• Added limitation note regarding -longoption syntax not being supported

[fdncred]

looks good, thanks! one small typo above.

[NotTheDr01ds]

Thanks! Fixed.

[fdncred]

Thanks