Merge pull request travis-ci#1407 from Junkern/patch-1

Clarifies `TRAVIS_BRANCH` when building for a tag.

Author: plaindocs

user/environment-variables.md

@@ -186,6 +186,7 @@ to tag the build, or to run post-build deployments.
   + for push builds, or builds not triggered by a pull request, this is the name of the branch.
   + for builds triggered by a pull request this is the name of the branch targeted by the pull
   request.
+  + for builds triggered by a tag, this is the same as the name of the tag (`TRAVIS_TAG`).
 
       > Note that for tags, git does not store the branch from which a commit was tagged.