Make BinarySearch() interface slightly more readable

Replace "equal" parameter with the (opposite) "lowerBound" one with a
more descriptive name, add a comment describing it and also remove the
default value to force explaining how it is used in the caller.

No real changes.

include/wx/arrstr.h

@@ -398,8 +398,10 @@ private:
   // (if the old buffer is big enough, just return NULL).
   wxString *Grow(size_t nIncrement);
 
-  // Binary search in the sorted array
-  size_t BinarySearch(const wxString& str, bool equal = false) const;
+  // Binary search in the sorted array: return the index of the string if it's
+  // present, otherwise, if lowerBound is true, return the position at which
+  // the string should be inserted and if it's false return wxNOT_FOUND.
+  size_t BinarySearch(const wxString& str, bool lowerBound) const;
 
   size_t  m_nSize,    // current size of the array
           m_nCount;   // current number of elements