pw_rpc: Update docs to reference FixedMtuChannelOutput
pw_rpc: Update docs to reference FixedMtuChannelOutput

Change-Id: If2a048d0d89d35ba2a8e5ed09db98e519790a11c
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/109376
Pigweed-Auto-Submit: Erik Gilling <1100d3bbb092e6691a138903fe8ea324306d03ea@google.com>
Reviewed-by: Anthony DiGirolamo <4cc40125edc24ec5bc72df7917ea0b403ee8cebe@google.com>
Commit-Queue: Auto-Submit <14637eda5603879df170705285bf30f5996692f0@pigweed.google.com.iam.gserviceaccount.com>

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