reuse-delta-base: improves some documentation

The current code got me a bit confused initially. So a bit more documentation
around it cannot hurt.

mercurial/configitems.py

@@ -2075,6 +2075,7 b' coreconfigitem('
     b'revlog.reuse-external-delta',
     default=True,
 )
+# This option is True unless `format.generaldelta` is set.
 coreconfigitem(
     b'storage',
     b'revlog.reuse-external-delta-parent',

mercurial/scmutil.py

@@ -1858,7 +1858,12 b' def gdinitconfig(ui):'
 
 
 def gddeltaconfig(ui):
-    """helper function to know if incoming delta should be optimised"""
+    """helper function to know if incoming deltas should be optimized
+
+    The `format.generaldelta` config is an old form of the config that also
+    implies that incoming delta-bases should be never be trusted. This function
+    exists for this purpose.
+    """
     # experimental config: format.generaldelta
     return ui.configbool(b'format', b'generaldelta')