The error exibits as build failures
of two types:

  1. extending_ghc.rst:: ERROR:
      Anonymous hyperlink mismatch:
        1 references but 0 targets.
      See "backrefs" attribute for IDs.

  2. reading sources... [ 33%] glasgow_exts
       Exception occurred:
          pickle.dump(doctree, f, pickle.HIGHEST_PROTOCOL)
       RecursionError: maximum recursion depth exceeded
       while pickling an object

Broken link created circular reference and failed to
serialize the result.

Fixed the problem by pointing to relevant section.

Signed-off-by: Sergei Trofimovich <siarheit@google.com>