Merge pull request travis-ci#3378 from travis-ci/am-tbt-98

[TBT-98] Update documentation for default env vars

Author: DominikAlberski

user/environment-variables.md

@@ -195,7 +195,7 @@ to tag the build, or to run post-build deployments.
 - `TRAVIS_BUILD_NUMBER`: The number of the current build (for example, "4").
 - `TRAVIS_BUILD_WEB_URL`: URL to the build log.
 - `TRAVIS_COMMIT`: The commit that the current build is testing.
-- `TRAVIS_COMMIT_MESSAGE`: The commit subject and body, unwrapped. PLEASE NOTE: This is not the same as *Custom Commit Message*.
+- `TRAVIS_COMMIT_MESSAGE`: The commit subject and body, unwrapped. It will contain *Custom Commit Message* if it was provided.
 - `TRAVIS_COMMIT_RANGE`: The range of commits that were included in the push
   or pull request. (Note that this is empty for builds triggered by the initial commit of a new branch.)
 - `TRAVIS_COMPILER`: Indicates the compiler used by the current job (e.g., `clang`, `gcc`).
@@ -223,6 +223,10 @@ to tag the build, or to run post-build deployments.
 - `TRAVIS_PULL_REQUEST_SLUG`:
   + if the current job is a pull request, the slug (in the form `owner_name/repo_name`) of the repository from which the PR originated.
   + if the current job is a push build, this variable is empty (`""`).
+- `TRAVIS_PULL_REQUEST_IS_DRAFT`:
+  + set to `true` if the current job is a pull request and the associated PR is in a draft state
+  + set to `false` if the current job is a pull request and the associated PR is not in a draft state
+  + if the current job is a push build, this variable is empty (`""`).
 - `TRAVIS_REPO_SLUG`: The slug (in form: `owner_name/repo_name`) of the repository currently being built.
 - `TRAVIS_SECURE_ENV_VARS`:
   + set to `true` if there are any encrypted environment variables.