webrtc: Fix signals documentation

Some GIR annotations were incorrect or even missing. The former isn't
good for bindings, while the latter is especially annoying for signal
handlers, as that means your arguments will get the wrong names in the
rendered documentation.
12 jobs for wip/nielsdg/fix-webrtc-docs in 44 minutes and 27 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Preparation
passed #290168
gst indent

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passed #290167
manifest

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  Build
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build fedora x86_64

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build nodebug fedora x86_64

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  Test
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check fedora

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passed #290172
integration testsuites fedora

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  Full Builds
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build cerbero android universal

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build cerbero cross win32

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build cerbero cross win64

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build cerbero fedora x86_64

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  Full Tests
passed #290177
valgrind bad

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  Apps
passed #290178
android universal examples

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