docs: clarify when MVCC snapshot is taken

Bruce Momjian <bruce@momjian.us>

Commit: 66c8040d509c86fd2f215c06eb16d82e570d3fb6
Author: Bruce Momjian <bruce@momjian.us>
Date: 2015-03-25T00:56:09Z
Releases: 9.5.0
docs:  clarify when MVCC snapshot is taken

Report by Álvaro Hernández Tortosa

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doc/src/sgml/mvcc.sgml modified +2 −1