doc fix: hg tags defaults to current parent revision, too.
Additionally use better language for the help text for tag,
cat and manifest.
default = 'changelog'
header = header.tmpl
changelog = changelog.tmpl
changelogentry = changelogentry.tmpl
filelog = filelog.tmpl
filelogentry = filelogentry.tmpl
tags = tags.tmpl
tagentry = tagentry.tmpl