util: buffer: Clarify scope of the escape operation in virBufferEscape

The escaping is applied only to the string, not the format argument.
State this fact in the docs.

src/util/virbuffer.c

@@ -505,8 +505,9 @@ virBufferEscapeSexpr(virBufferPtr buf,
  * @str: the string argument which needs to be escaped
  *
  * Do a formatted print with a single string to a buffer.  Any characters
- * in the provided list are escaped with the given escape.  Auto indentation
- * may be applied.
+ * in the provided list that are contained in @str are escaped with the
+ * given escape.  Escaping is not applied to characters specified in @format.
+ * Auto indentation may be applied.
  */
 void
 virBufferEscape(virBufferPtr buf, char escape, const char *toescape,