doc/groff.texi: Revise vertical spacing material.

* Document text baseline location as a property of the output line.
* Recast description of `sp` request entirely, describing more aspects
  of its behavior.  Add forward cross references to implicated topics.
  Drop some material about trap interaction spread amid examples since we now handle that
  both earlier, in the initial presentation, and later, via cross
  references to the presentation of vertical position traps.
* Recast description of `ls` request and `.L` register.
* Recast description of `\x` escape sequence and `.a` register.  Preview
  the terminology that occurs again later when the `vs` and `pvs`
  requests are presented.  Say more to motivate the existence of these
  features.
* Recast description of `ns` and `rs` requests and `.ns` register.
* Shift Texinfo @codequote* commands, marking the section/node
  "Manipulating Spacing" as reviewed for ` and ' glyph usage in
  examples.

Thanks to Dave Kemper for compelling me to review this subject.

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