doc: Document order of define_peephole2 scanning

I was a bit surprised when my newly-added define_peephole2 didn't
match, but it was because it was expected to partially match the
generated output of a previous define_peephole2, which matched and
modified the last insn of a sequence to be matched.  I had assumed
that the algorithm backed-up the size of the match-buffer, thereby
exposing newly created opportunities *with sufficient context* to all
define_peephole2's.  While things can change in that direction, let's
start with documenting the current state.

	* doc/md.texi (define_peephole2): Document order of scanning.

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