docs: Corrects or expands documentation comments for various blocks where I have had to read the source code to understand the

- char_to_short and short_to_char scale by a factor of 1<<8.
- What delay actually does.
- File descriptor blocks close their FDs when destroyed.

And one code change:

The firdes transition width check error text is made consistent with other similar messages.

Addresses Issue #686.
filter: upping the allowed relative error in freq_xlating_fft_filter's QA from 0.001 to 0.0012 to account for minor precision errors with AVX.
Removing trailing/extra whitespaces before release.

We should be more careful about letting these into the code in the future. In emacs, we can use (add-hook 'before-save-hook 'delete-trailing-whitespace).