#434 Document "change label for subsegments"

[Jean-Christophe Helary]

https://github.com/omegat-org/omegat/pull/1224

Display first entry as file name instead of empty string

Adds new variables for "comes from":

• ${matchSource} contains a string like 'From TM'
• ${matchSource-dingbat} contains a symbol, better than ${matchSource} when added in a very long line
• ${matchSource-emoji} contains an emoji, better than ${matchSource} but may be unavailable in some fonts