qtconf: Add comment to clarify code handling env vars

The string handling is not immediately clear, unless one already knows
what it is supposed to do.

Change-Id: Icd5746513d760d478f03c8aded3669b06b692a87
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Ahmad Samir <a.samirh78@gmail.com>
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

src/corelib/global/qlibraryinfo.cpp

@@ -554,6 +554,10 @@ QString QLibraryInfoPrivate::path(QLibraryInfo::LibraryPath p, UsageMode usageMo
             }
 
             qsizetype startIndex = 0;
+            /* We support placeholders of the form $(<ENV_VAR>) in qt.conf.
+               The loop below tries to find all such placeholders, and replaces
+               them with the actual value of the ENV_VAR environment variable
+             */
             while (true) {
                 startIndex = ret.indexOf(u'$', startIndex);
                 if (startIndex < 0)