A clean-up task: delete "Use scratch_pool for temporary allocations" from doc strings

No particular time frame, but a little clean-up task for
anybody who has a touch of OCD or likes
playing with regular expression search and replace, would be to remove
the clause"Use scratch_pool for temporary allocations" from all our doc strings.
Documentation should be helpful and concise; repeating universal truths is neither.  Our 'scratch_pool' convention is universal.  Not all code uses the convention, but it always means the same thing when it is used.

(The 'result_pool' convention, on the other hand, is a bit more involved.  Results are not always allocated in result_pool, and there are often conditions attached such as "except some of the values in the result structure are shallow-copied from the inputs".  So the public APIs at least should probably always be explicit about use of the result pool.)

- Julian

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