gropdf(1): Fix more content/style/markup nits.

Content:
* More carefully distinguish escape sequences from device control
  commands; the former are more general.
* The *roff `\X` escape sequence is a device control command, not a
  "special".  *roff already freights the latter term with enough meaning
  without hauling in TeX's use for it.
* The "ps:" or "pdf:" part of a device control command escape sequence
  parameter is the "tag", not the entire parameter.  It is a name
  spacing convention.
* Clarify operation of `xrev` device control command.

Style:
* Recast: all device control commands are "extensions"; there is no
  standard set.
* Convert mentions of (g)pic(1) and grops(1) to man page cross
  references.
* Set pic(1) command names in bold, not italics.
* Set "mom" macro package name in italics, not bold.
* Set "gropdf" in italics, not bold.
* Capitalize "PDF" in prose.
* Protect Perl's "localtime" function name from hyphenation.
* Hyphenate adjectival phrases.
* Hyphenate metasyntactic variable consistently with its introduction.
* Otherwise recast for (attempted) clarity and concision.

Markup:
* Use two empty requests where vertical space is expected.
* Annotate a mismatch in conventions between grotty(1) and gropdf(1).

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