Buffer: add comment for clamp

src/buffer.hh

@@ -137,6 +137,7 @@ public:
     BufferIterator iterator_at(const BufferCoord& line_and_column) const;
     BufferCoord    line_and_column_at(const BufferIterator& iterator) const;
 
+    // returns nearest valid coordinates from given ones
     BufferCoord    clamp(const BufferCoord& line_and_column) const;
 
     const std::string& name() const { return m_name; }