Doc: capitalize TS and QM

That are abbreviations of "translation source" and
"Qt message". Add the full forms, too.

Change-Id: I983ec829adfa694781b2faba82c05ef18ef04cce
Reviewed-by: Leena Miettinen <riitta-leena.miettinen@digia.com>

doc/src/howto/creator-external-tools.qdoc

@@ -43,11 +43,12 @@
     code and translations. The lrelease tool is used to create run-time
     translation files for use by the released application.
 
-    To synchronize ts files from a translator with the application code,
+    To synchronize translation source (TS) files from a translator with the
+    application code,
     select \gui {Tools > External > Text > Linguist > Update Translations
     (lupdate)}.
 
-    To generate from the ts files qm translation files that can be used by an
+    To generate from the TS files Qt message (QM) files that can be used by an
     application, select \gui {Tools > External > Text > Linguist > Release
     Translations (lrelease)}.