compute-spectrogram-feats documentation, etc.

[danpovey]

The binary compute-spectrogram-feats.cc is not style compliant and the documentation is not good [not clear what spectrogram features are; is it a power spectrum, a log power spectrum?]

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically closed by a bot strictly because of inactivity. This does not mean that we think that this issue is not important! If you believe it has been closed hastily, add a comment to the issue and mention @kkm000, and I'll gladly reopen it.