libsframe/doc: fix some warnings

libsframe/doc: fix some warnings

'make pdf' in libsframe shows some warnings, some of which (especially
the Overfull warnings) are causing undesirable effects on the rendered
output.  Few examples of the warnings:

  Underfull \hbox (badness 10000) in paragraph at lines 406--407
   @texttt pauth_

  Underfull \hbox (badness 10000) in paragraph at lines 407--410
   @textrm Specify which key is used for signing the return

  ...

  Overfull \hbox (2.0987pt too wide) in paragraph at lines 412--413
   @texttt fdetype[]|

  ...

  Overfull \hbox (28.87212pt too wide) in paragraph at lines 446--447
   @textrm SFRAME[]FDE[]TYPE[]PCMASK|

  ...

This patch adjusts column widths of the affected cells to fix a subset
of these warnings.  For the rest of the warnings, use explicit newline
command to fix them.

libsframe/
	* doc/sframe-spec.texi: Fix various underfull and overfull
	warnings.