Add a simple example application for the spectrum element, include it in the docs...

Add a simple example application for the spectrum element, include it in the docs, and fix some documentation ambigui...

Original commit message from CVS:
* docs/plugins/Makefile.am:
* gst/spectrum/gstspectrum.c:
* tests/examples/spectrum/.cvsignore:
* tests/examples/spectrum/Makefile.am:
* tests/examples/spectrum/spectrum-example.c:
Add a simple example application for the spectrum element, include it
in the docs, and fix some documentation ambiguities.
Fixes: #348085