Update .abi-compliance-history file.

As foretold by commit a72f7d97be, this commit moves the baseline
point for ABI compatibility for v18 to commit c8af5019be.  While at
it, add some more commentary and adjust the format of the entries
to improve both human and machine readability.  There's a good
chance we'll add an .abi-compliance-history file to the other
back-branches, but for now this effort is limited to v18.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Peter Geoghegan <pg@bowt.ie>
Reviewed-by: David E. Wheeler <david@justatheory.com>
Reviewed-by: Mankirat Singh <mankiratsingh1315@gmail.com>
Discussion: https://postgr.es/m/aPJ03E2itovDBcKX%40nathan
Backpatch-through: 18 only

.abi-compliance-history

@@ -1,7 +1,36 @@
 # Reference point for ABI compliance checks
-# Add new entries by adding the output of the following to the top of the file:
-# $ git log --pretty=format:"%H # %cd%n# %s" $ABIBREAKGITHASH -1 --date=iso
 #
-# Initial entry would normally point to a commit slightly before the .0 stamp.
-9bbcec6030a2744d83311370ec92213fbd76e514 # 2025-09-22 14:18:56 +0200
+# This file lists commits on the current branch that break ABI compatibility in
+# ways that have been deemed acceptable (e.g., removing an extern function with
+# no third-party uses).  The primary intent of this file is to control the ABI
+# compliance checks on the buildfarm, but it also serves as a central location
+# to document the justification for each.
+#
+# In general, entries should be added reactively after an abi-compliance-check
+# buildfarm failure.  It is important to verify the details of the breakage
+# match expectations, as the first entry listed will become the updated ABI
+# baseline point.
+#
+# Add new entries by adding the output of the following to the top of the file:
+#
+# $ git log --pretty=format:"%H%n#%n# %s%n# %cd%n#%n# <ADD JUSTIFICATION HERE>" $ABIBREAKGITHASH -1 --date=iso
+#
+# Be sure to replace "<ADD JUSTIFICATION HERE>" with details of your change and
+# why it is deemed acceptable.
+
+c8af5019bee5c57502db830f8005a01cba60fee0
+#
+# Fix lookups in pg_{clear,restore}_{attribute,relation}_stats().
+# 2025-10-15 12:47:33 -0500
+#
+# This commit replaced two functions related to lookups/privilege checks for
+# the new stats stuff in v18 with RangeVarGetRelidExtended().  These functions
+# were not intended for use elsewhere, exist in exactly one release (18.0), and
+# do not have any known third-party callers.
+
+9bbcec6030a2744d83311370ec92213fbd76e514
+#
 # Translation updates
+# 2025-09-22 14:18:56 +0200
+#
+# This is the original ABI baseline point for REL_18_STABLE.