find.1: improve table formatting

* find/find.1:
- Use paragraphing macros to put vertical space above and below the
  tables (where not already present) so that they looks more like
  typographical "displays".[1]
- Simplify "XY" table definition (see groff's tbl(1) man page).  This
  simpler definition has the same result (and is more adaptable in the
  event the text of the table is revised).
- Put horizontal rule below column headings in table that uses them.
- Use thin space escape sequences `\|` between the dots of an ellipsis;
  the effect is visible only on typesetting devices.[2]  I left the
  dummy character escape sequence `\&` trailing it intact, but observe
  for the edification of nit-pickers that it has no effect in this
  context.[3]

[1] man(7) implementations are historically inconsistent with respect to
    whether a `TS` macro call should put any vertical space on the
    output, and compositional practices on the part of man page authors
    have thus been, to quote Ingo Schwarze, "wildly inconsistent".

    https://savannah.gnu.org/bugs/?62841

[2] groff_man_style(7):

       Examples of ellipsis usage are shown above, in subsection
       “Synopsis macros”.  The idiomatic roff ellipsis is three dots
       (periods) with thin space escape sequences \| internally
       separating them.

[3] tbl(1):

     Ordinarily, a table entry is typeset rigidly.  It is not filled,
     broken, hyphenated, adjusted, or populated with supplemental inter‐
     sentence space.

Discussed at:
https://lists.gnu.org/r/bug-findutils/2025-11/msg00094.html

Copyright-paperwork-exempt: Yes

find/find.1

@@ -937,18 +937,16 @@ Fails if the timestamps are
 .BR equal .
 The letters \fIX\fR and \fIY\fR can be any of the following letters:
 .
+.IP
 .TS
-ll
-ll
-ll
-ll
-llw(2i).
+LL.
 a	The access time of the file \fIreference\fR
 B	The birth time of the file \fIreference\fR
 c	The inode status change time of \fIreference\fR
 m	The modification time of the file \fIreference\fR
 t	\fIreference\fR is interpreted directly as a time
 .TE
+.IP
 .
 Some combinations are invalid; for example, it is invalid for
 .I X