docs: clarify separation between plpgsql-parser and plpgsql-deparser #943

Job	Run time
parser-tests (pgsql-cli)	57s
parser-tests (proto-parser)	56s
parser-tests (plpgsql-parser)	1m 4s
parser-tests (transform)	5m 43s
parser-tests (deparser)	59s
parser-tests (traverse)	1m 0s
parser-tests (plpgsql-deparser)	1m 7s
parser-tests (utils)	59s
parser-tests (parser)	58s
	13m 43s

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