bug#18207: erc-truncate-buffer-on-save: misleading variable name and docstring

[Ivan Shmakov <ivan@siamics.net>]

Package:  emacs
Severity: minor

	The docstring for erc-truncate-buffer-on-save reads:

Truncate any ERC (channel, query, server) buffer when it is saved.

	This is somewhat misleading, as there’re also erc-truncate
	module, and, in particular, – the erc-truncate-buffer function,
	which are used to truncate buffer /to a given size/ (see
	erc-max-buffer-size.)  On the contrary, this variable, when
	non-nil, forces the buffer to be /completely erased,/ as per the
	following lisp/erc/erc-log.el fragment:

   434		      (if (and erc-truncate-buffer-on-save
   435			       (called-interactively-p 'interactive))
   436			  (progn
   437			    (let ((inhibit-read-only t)) (erase-buffer))
   438			    (move-marker erc-last-saved-position (point-max))
   439			    (erc-display-prompt))

	I believe that this behavior should be made clear in the
	documentation, like:

Erase ERC (channel, query, server) buffers when one is saved.

This function uses `erase-buffer' and has nothing to do with the
`erc-truncate-buffer' function.  In order to keep ERC buffer sizes
within a specific limit, you may set `erc-max-buffer-size' and use
`erc-truncate-mode' instead.

	It also makes sense to rename this variable (leaving a
	compatibility alias.)

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