Clean up and document the API for XLogOpenRelation and XLogReadBuffer.

Tom Lane <tgl@sss.pgh.pa.us>

Commit: 6d61cdec0761dc1f86a196899825b52b1186d00e
Author: Tom Lane <tgl@sss.pgh.pa.us>
Date: 2006-03-29T21:17:39Z
Releases: 8.2.0
Clean up and document the API for XLogOpenRelation and XLogReadBuffer.
This commit doesn't make much functional change, but it does eliminate some
duplicated code --- for instance, PageIsNew tests are now done inside
XLogReadBuffer rather than by each caller.
The GIST xlog code still needs a lot of love, but I'll worry about that
separately.

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