Issue #6953: Rearrange and expand Readline module documentation

* Group functions into six new subsections
* Document the underlying Readline function or variable accessed
* get_history_length() returns the history file limit
* clear_history() is conditionally compiled in
* Clarify zero and one bases for history item indexes
* parse_and_bind() uses its argument directly as an init line
* Change "command line" to "line buffer" for consistency
* read_init_file() also executes the file
* read_history_file() replaces the previous history
* write_history_file() overwrites any existing file
* Differentiate history file lines from history list items, which could be
  multi-line
* Add more information about completion, also addressing Issue #10796
* libedit (Editline) may be used on any platform; detection is OS X specific

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