rtpbin: Improve RTPStorage action signal documentation

This is a tiny clarification as the storage was loosely named "storage".
This change clarify that the storage is specificaly used for received RTP
packets. This is unlike the storage found in rtprtxsend that stores a
backlog of sent RTP packets.
8 jobs for <span class="ref-name">doc-rtpbin-storage</span> in 12 minutes and 36 seconds (queued for 1 second)
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manifest

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  Build
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build fedora x86_64

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build android arm64 api28

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build cerbero cross win32

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