bug#4654: 23.1; Elisp manual doc of abbreviate-file-name

[Drew Adams]

The doc string mentions this qualification, but the Elisp manual does
not: " (unless the home directory is a root directory)".
 
Please mention this in the manual also.
 
Question: Why? What is the rationale for not substituting `~' when it
is a root directory? If the reason is short to express, perhaps it
should be included in the doc. Understanding the rationale helps one
remember what the function does.
 
In GNU Emacs 23.1.1 (i386-mingw-nt5.1.2600)
 of 2009-07-29 on SOFT-MJASON
Windowing system distributor `Microsoft Corp.', version 5.1.2600
configured using `configure --with-gcc (4.4)'