bug#2515: 23.0.91; Documentation of clear-abbrev-table seems wrong

[Leo]

Please write in English if possible, because the Emacs maintainers
usually do not have translators to read other languages for them.

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Please describe exactly what actions triggered the bug
and the precise symptoms of the bug:

I came across:

,----[ (info "(elisp)Abbrev Tables") ]
|  -- º¯Êý: clear-abbrev-table table
|      This function undefines all the abbrevs in abbrev table TABLE,
|      leaving it empty.  It always returns `nil'.
`----

However after experimenting with it, I have the impression that it
returns the number of abbrevs being cleared.

Please take a look at this.

Leo