help: align description of 'base rev' with reality [issue5488]

The text about revlogs seems to be wrong about -1 being used to indicate
the start of a delta chain. Attempt to correct this.

mercurial/help/internals/revlogs.txt

@@ -108,9 +108,9 b' 12-15 (4 bytes)'
 
 16-19 (4 bytes)
    Base or previous revision this revision's delta was produced against.
-   -1 means this revision holds full text (as opposed to a delta).
-   For generaldelta repos, this is the previous revision in the delta
-   chain. For non-generaldelta repos, this is the base or first
+   This revision holds full text (as opposed to a delta) if it points to
+   itself. For generaldelta repos, this is the previous revision in the
+   delta chain. For non-generaldelta repos, this is the base or first
    revision in the delta chain.
 
 20-23 (4 bytes)