add a character and a string definitions for an ellipsis

  Seen in contrib/rfc1345.tmac as

.char \[,.] \[u2026]\"     HORIZONTAL ELLIPSIS (Vim 8.0.0062)

  Seen in tmac/an.tmac as

.  ds an-ellipsis \|.\|.\|.\|\"

  Suggestions:

  Define as a fallback

.char \[,.] \|.\|.\|.\|

  or

.char \[,.] \&.\|.\|.

  Define as a short string

.ds El \|.\|.\|.\|\"

  or

.ds El \&.\|.\|.\"

  The string definition can be copied to the file that uses ellipsis,
replacing '...'.

The width of UCS (Universal Character Set) 2026 is too short for my taste.

-.-

  Many examples of man pages in groff contain '.\|.\|.' with additional
'\&' to ensure that the formatter interprets the string correctly.

-.-

  I found neither '\*(El' nor '\*[El' strings in my man[1-8] directories
from Debian.
So these are not common there.