dubious phrase in on-line help for skip-chars-forward

[Jonas Jarnestrom]

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In GNU Emacs 20.6.1 (sparc-sun-solaris2.6, X toolkit)
 of Fri Mar  3 2000 on iris
configured using `configure  --with-gcc --with-pop --with-x-toolkit=lucid 
--bindir=/usr/local/bin --libexecdir=/usr/local/libexec'


In skip-chars-forward's doc-string it says:

STRING is like the inside of a `[...]' in a regular expression
except that `]' is never special and `\' quotes `^', `-' or `\'
==> (but not as the end of a range; quoting is never needed there). <==

As far as I can see, this assertion doesnt always hold for `-'.

If I have a buffer string like:
sdfsdfsd-
I can skip over it with (skip-chars-forward "a-z-")
i.e. dash doesnt need quoting. 
However, if I have an expr like (skip-chars-forward "a-zA-Z0-9_.-")
it stops at the `-'

It seems as if I have to quote the trailing dash to get what I want:
(skip-chars-forward "a-zA-Z0-9_.\\-")
which according to the on-line doc, should not be needed at the end of
the range.

(skip-chars-backward has the same behavior)