Better document tag trigger

Tag trigger has been introduced in more recent versions of Bamboo, however it's not documented in specs docs, and there's no info on which env vars can provide tag name, revision and things related to a git tag. Tag trigger does not not mention such env vars as:

${bamboo.TagBuildTriggerReason.buildKey}
${bamboo.TagBuildTriggerReason.customRevision}
${bamboo.TagBuildTriggerReason.tagName}