Fix error in documentation.

HACKING

@@ -175,7 +175,6 @@ systems) it occupies two elements. The following meta items occupy just one
 element, with no data:
 
 META_ACCEPT           (*ACCEPT)
-META_ALT              | alternation 
 META_ASTERISK         *
 META_ASTERISK_PLUS    *+
 META_ASTERISK_QUERY   *?
@@ -216,11 +215,15 @@ relevant.
 The following have data in the lower 16 bits, and may be followed by other data
 elements:
 
+META_ALT              | alternation
 META_BACKREF
 META_CAPTURE
 META_ESCAPE
 META_RECURSE
 
+If the data for META_ALT is non-zero, it is inside a lookbehind, and the data
+is the length of its branch, for which OP_REVERSE must be generated.
+
 META_BACKREF, META_CAPTURE, and META_RECURSE have the capture group number as
 their data in the lower 16 bits of the element.
 
@@ -275,13 +278,18 @@ The following are followed just by an offset, for use in error messages:
 
 META_COND_ASSERT      (?(?assertion)
 META_COND_DEFINE      (?(DEFINE)
-META_LOOKBEHIND       (?<= 
-META_LOOKBEHINDNOT    (?<!
 
 In fact, META_COND_ASSERT is used for any group starting (?( that does not
 match any of the other META_COND cases. The check that this group is an
 assertion (optionally preceded by a callout) happens at compile time.
 
+The following are also followed just by an offset, but also the lower 16 bits
+of the main word contain the length of the first branch of the lookbehind
+group; this is used when generating OP_REVERSE for that branch.
+
+META_LOOKBEHIND       (?<=
+META_LOOKBEHINDNOT    (?<!
+
 The following are followed by two values, the minimum and maximum. Repeat
 values are limited to 65535 (MAX_REPEAT). A maximum value of "unlimited" is
 represented by UNLIMITED_REPEAT, which is bigger than MAX_REPEAT: