docs(coding): fix string usage guidelines

Update docs to recommend using `%` operator with `QLatin1String` even for joining two strings (not only *more than two* strings).

Mentioned optimisation of "lightweight" QLatin1String copying-and-converting-to-utf16 when appending only works with QStringBuilder (operator %). Otherwise, QLatin1String will be converted to QString, and only then copied (appended), which is slower.

doc/coding_standards.md

@@ -489,23 +489,23 @@ temporary QString and thus avoids malloc:
         handleToxSave(firstParam.toUtf8());
 ```
 
-* Use `QLatin1String` when appending to string, joining two strings.
+* Use `QStringBuilder` and `QLatin1String` when joining strings (and chars)
+together.
 
-QLatin1String is literal type and knows string length at compile time
+`QLatin1String` is literal type and knows string length at compile time
 (compared to `QString(const char*)` run-time cost with plain C++
 string literal). Also, copying 8-bit latin string requires less memory
 bandwith compared to 16-bit `QStringLiteral` mentioned earlier, and
-copying here is unavoidable (and thus `QStringLiteral` loses it's purpose):
+copying here is unavoidable (and thus `QStringLiteral` loses it's purpose).
+
+Include `<QStringBuilder>` and use `%` operator for optimized single-pass
+concatenation with help of expression template's lazy evaluation:
+
 ```
-        if (!dir.rename(logFileDir + QLatin1String("qtox.log"),
-                        logFileDir + QLatin1String("qtox.log.1")))
+        if (!dir.rename(logFileDir % QLatin1String("qtox.log"),
+                        logFileDir % QLatin1String("qtox.log.1")))
             qCritical() << "Unable to move logs";
 ```
-
-* Use `QStringBuilder` when joining more than two strings (and chars) together.
-
-Include `<QStringBuilder>` and use `%` operator for optimized single-pass
-concatination with help of expression template's lazy evaluation:
 ```
     QCommandLineParser parser;
     parser.setApplicationDescription(QLatin1String("qTox, version: ")
@@ -513,7 +513,8 @@ concatination with help of expression template's lazy evaluation:
     % QLatin1String(__TIME__) % QLatin1Char(' ') % QLatin1String(__DATE__));
 ```
 
-* Use `QLatin1Char` to avoid UTF-16-char handling (same as in previous example):
+* Use `QLatin1Char` to avoid UTF-16-char handling (same as in previous
+example):
 ```
     QString path = QString(__FILE__);
     path = path.left(path.lastIndexOf(QLatin1Char('/')) + 1);