document extra sed 's' behaviour

[Dan Jacobson]

These appear not to be documeted

$ echo 1x1x|sed 's/1/a/;s'
axx

$ echo 1x1x|sed 's/1/a/;s///'
axx

BTW, after an s, even ; and newline are seen as separators belonging
to s.  So maybe emphasize that in this doc part:
`s/REGEXP/REPLACEMENT/FLAGS'
     (The `/' characters may be uniformly replaced by any other single
     character within any given `s' command.)
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