Adds documentation for 'Play all manual' button

See merge request gitlab-org/gitlab-ce!28502

(cherry picked from commit 7a9a65e5)

32a99c6c Adds documentation for stage button
4ec9f042 Apply suggestion to doc/ci/pipelines.md
062844cc Apply suggestion to doc/ci/pipelines.md
ecc7b72a Apply suggestion to doc/ci/pipelines.md
c4bcf858 Apply suggestion to doc/ci/pipelines.md
b0b16210 Apply suggestion to doc/ci/pipelines.md