pgsql: Add a comment warning against use of pg_usleep() for long sleeps

Add a comment warning against use of pg_usleep() for long sleeps.

Follow-up to commit 873ab97219caabeb2f7b390268a4fe01e2b7518c, in which
I noted that WaitLatch was a better solution in the commit log message,
but neglected to add any documentation in the code.

Branch
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master

Details
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http://git.postgresql.org/pg/commitdiff/8c1a71d36f5d667f3c2236e0e015a48f809ca240

Modified Files
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src/port/pgsleep.c | 10 ++++++++++
1 file changed, 10 insertions(+)

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