Merge pull request travis-ci#3376 from travis-ci/tbt-39-job-restarted-variables

TBT-39 Add job restarted env variables in docs

Author: murtaza-swati

user/environment-variables.md

@@ -231,6 +231,10 @@ to tag the build, or to run post-build deployments.
 - `TRAVIS_TEST_RESULT`: **0** if all commands in the `script` section (up to the point this environment variable is referenced) have exited with zero; **1** otherwise.
 - `TRAVIS_TAG`: If the current build is for a git tag, this variable is set to the tag's name, otherwise it is empty (`""`).
 - `TRAVIS_BUILD_STAGE_NAME`: The [build stage](/user/build-stages/). If a build does not use build stages, this variable is empty (`""`).
+- `TRAVIS_JOB_RESTARTED`:
+  + set to `true` if the build is restarted.
+  + set to `false` if the build has not been restarted.
+- `TRAVIS_JOB_RESTARTED_BY`: The login of the user triggering the restart.
 
 Language-specific builds expose additional environment variables representing
 the current version being used to run the build. Whether or not they're set