Обсуждение: pgsql: Doc: improve explanation of how to use our code coverage infrast

Doc: improve explanation of how to use our code coverage infrastructure.

The reference to running "make coverage" in a subdirectory was a
bit obscure, so clarify what happens when you do that.  Do a little
desultory copy-editing, too.

Per a question from Peter Smith.

Discussion: https://postgr.es/m/CAHut+Pu0r3AjRSyu5E0v2-zRj8r24OSrkWs3fEBxOuaw1i8DKA@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/8d132b2850d4c95ed5666a09b295dea645bbc884

Modified Files
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doc/src/sgml/regress.sgml | 21 ++++++++++++++++-----
1 file changed, 16 insertions(+), 5 deletions(-)