std.regex: document backreferences in the syntax table

The pattern syntax table listed named groups and referred to "direct
numbers" in passing, but had no entry for backreferences themselves,
making them look unsupported. Add a row describing `\1`, `\2`, ...,
including the multi-digit case.

Author: kubo39

std/regex/package.d

@@ -211,6 +211,11 @@ They met on 24/01/1970.
         When referring to a matched portion of text,
         names work like aliases in addition to direct numbers.
      )
+    $(REG_ROW \1$(COMMA) \2 ..., Backreference: matches the same text most
+        recently captured by the capturing group with that number. Groups are
+        numbered from 1 by the position of their opening parenthesis. A
+        multi-digit reference such as \12 denotes group 12 when at least that
+        many capturing groups precede it. )
     $(REG_TITLE Assertions, Match position rather than character )
     $(REG_ROW ^, Matches at the beginning of input or line (in multiline mode).)
     $(REG_ROW $, Matches at the end of input or line (in multiline mode). )