Fix documentation for "pool" object in gn binary.

Followup to comments on https://codereview.chromium.org/2006923004/.

BUG=612786

Review-Url: https://codereview.chromium.org/2045713003
Cr-Original-Commit-Position: refs/heads/master@{#398400}
Cr-Mirrored-From: https://chromium.googlesource.com/chromium/src
Cr-Mirrored-Commit: e9efe32c54352ee81afac228219361eb77c832cf