Minor comment fixes, no code changes:

  - 'sub request' -> 'subrequest'
  - @retrn -> @return
  - ...

PR:
Obtained from:
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git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/APACHE_2_0_BRANCH@105409 13f79535-47bb-0310-9956-ffa450edef68