diff doc obsolete warning too late

[Dan Jacobson]

looking at the diff Info page,
`-LINES'
     Show LINES (an integer) lines of context.  This option does not
     specify an output format by itself; it has no effect unless it is
     combined with `-c' (*note Context Format::.) or `-u' (*note
     Unified Format::.).  This option is obsolete.  For proper
     operation, `patch' typically needs at least two lines of context.

Well, jeez, if the option is obsolete why don't you come out and say
so at the top of the paragraph instead of us wasting brain cells
storing the info until we run into the warning.
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