Update override comment with a full explanation of what it does

Also checks for original url emptiness before returning it.

python/server/auto_generated/qgsfcgiserverrequest.sip.in

@@ -38,9 +38,11 @@ Returns true if an error occurred during initialization
 
 :return: the request url
 
-Subclasses may override in case the original URL needs to be
+Overrides base implementation because FCGI is typically behind
-returned instead of the rewritten one (i.e. from a web server
+a proxy server and QGIS Server will see a rewritten QUERY_STRING.
-rewrite module).
+FCGI implementation stores the REQUEST_URI (which is the URL seen
+by the proxy before it gets rewritten) and returns it instead of
+the rewritten one.
 %End
 
 

src/server/qgsfcgiserverrequest.cpp

@@ -135,7 +135,7 @@ QByteArray QgsFcgiServerRequest::data() const
 
 QUrl QgsFcgiServerRequest::url() const
 {
-  return mOriginalUrl;
+  return mOriginalUrl.isEmpty() ? QgsServerRequest::url() : mOriginalUrl;
 }
 
 // Read post put data

src/server/qgsfcgiserverrequest.h

@@ -45,9 +45,11 @@ class SERVER_EXPORT QgsFcgiServerRequest: public QgsServerRequest
     /**
      * \returns  the request url
      *
-     * Subclasses may override in case the original URL needs to be
+     * Overrides base implementation because FCGI is typically behind
-     * returned instead of the rewritten one (i.e. from a web server
+     * a proxy server and QGIS Server will see a rewritten QUERY_STRING.
-     * rewrite module).
+     * FCGI implementation stores the REQUEST_URI (which is the URL seen
+     * by the proxy before it gets rewritten) and returns it instead of
+     * the rewritten one.
      */
     QUrl url() const override;