Documentation: mutually link update-ref and symbolic-ref

These two commands are similar enough to acknowledge each other on their
documentation pages.

See the previous commit where we discussed that option-less update-ref
does not support updating symbolic refs but symbolic-ref does.

Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Taylor Blau <me@ttaylorr.com>

Documentation/git-symbolic-ref.txt

@@ -73,6 +73,10 @@ default.
 symbolic ref were printed correctly, with status 1 if the requested
 name is not a symbolic ref, or 128 if another error occurs.
 
+SEE ALSO
+--------
+linkgit:git-update-ref[1]
+
 GIT
 ---
 Part of the linkgit:git[1] suite

Documentation/git-update-ref.txt

@@ -190,6 +190,10 @@ otherwise it will just try to read them and update them as a regular
 file (i.e. it will allow the filesystem to follow them, but will
 overwrite such a symlink to somewhere else with a regular filename).
 
+SEE ALSO
+--------
+linkgit:git-symbolic-ref[1]
+
 GIT
 ---
 Part of the linkgit:git[1] suite