help: don't let tools reflow revlog flags list

Before this change, the text about revlog flags was reflowed into a
single paragraph, which made it a bit hard to read. I don't even know
the rules around this, but adding a blank line before each flag seems
to prevent the reflowing.

mercurial/help/internals/revlogs.txt

@@ -91,7 +91,9 b' 0-5 (6 bytes)'
 
 6-7 (2 bytes)
    Bit flags impacting revision behavior. The following bit offsets define:
+
    0: REVIDX_ISCENSORED revision has censor metadata, must be verified.
+
    1: REVIDX_EXTSTORED revision data is stored externally.
 
 8-11 (4 bytes)