NaCl: Expand comment about PpapiHostMsg_OpenResource in ppapi_messages.h
NaCl: Expand comment about PpapiHostMsg_OpenResource in ppapi_messages.h

BUG=302078
TEST=none

Review URL: https://codereview.chromium.org/1540953002

Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#366543}

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