help: use a single paragraph to describe full and abbreviated nodeids

The texts describing 40-digit strings and the abbreviated form are
closely related, so make it a single paragraph.

mercurial/help/revisions.txt

@@ -4,9 +4,7 b' A plain integer is treated as a revision'
 treated as sequential offsets from the tip, with -1 denoting the tip,
 -2 denoting the revision prior to the tip, and so forth.
 
-A 40-digit hexadecimal string is treated as a unique revision
-identifier.
-
+A 40-digit hexadecimal string is treated as a unique revision identifier.
 A hexadecimal string less than 40 characters long is treated as a
 unique revision identifier and is referred to as a short-form
 identifier. A short-form identifier is only valid if it is the prefix

tests/test-help.t

@@ -1165,7 +1165,6 b' Test a help topic'
       denoting the revision prior to the tip, and so forth.
   
       A 40-digit hexadecimal string is treated as a unique revision identifier.
-  
       A hexadecimal string less than 40 characters long is treated as a unique
       revision identifier and is referred to as a short-form identifier. A
       short-form identifier is only valid if it is the prefix of exactly one
@@ -2781,10 +2780,7 b' Dish up an empty repo; serve it cold.'
   -2 denoting the revision prior to the tip, and so forth.
   </p>
   <p>
-  A 40-digit hexadecimal string is treated as a unique revision
-  identifier.
-  </p>
-  <p>
+  A 40-digit hexadecimal string is treated as a unique revision identifier.
   A hexadecimal string less than 40 characters long is treated as a
   unique revision identifier and is referred to as a short-form
   identifier. A short-form identifier is only valid if it is the prefix