blocks: adding documentation for vector_sink's reserve_items param

Author: marcusmueller

gr-blocks/include/gnuradio/blocks/vector_sink_X.h.t

@@ -41,6 +41,15 @@ namespace gr {
       // gr::blocks::@NAME@::sptr
       typedef boost::shared_ptr<@NAME@> sptr;
 
+      /*!
+       * \brief Make a new instance of the vector source, and return a shared pointer to it.
+       * \param vlen length of vector items
+       * \param reserve_items reserve space in the internal storage for this many items;
+       *                      the internal storage will still grow to accommodate more item
+       *                      if necessary, but setting this to a realistic value can avoid
+       *                      memory allocations during runtime, especially if you know a
+       *                      priori how many items you're going to store.
+       */
       static sptr make(const int vlen = 1, const int reserve_items = 1024);
 
       //! Clear the data and tags containers.