Improve docs on `n_neighbors` relation with `search_window_size`

[mihaic]

The C++ docs describe the relationship well:

If the current value of get_search_window_size() is less than num_neighbors, it will temporarily be set to num_neighbors.
https://intel.github.io/ScalableVectorSearch/cpp/internal/index_impl/vamana.html#_CPPv4I0_N4data22ImmutableMemoryDatasetEEN3svs5index6vamana11VamanaIndex6searchEv15QueryResultViewI1IERK7QueriesRK22search_parameters_typeRKN3lib16DefaultPredicateE

We should use that description in the other places where num_neighbors and search_window_size are discussed, e.g., set_search_window_size:
https://intel.github.io/ScalableVectorSearch/cpp/index/dynamic_vamana.html#_CPPv4N3svs13DynamicVamana22set_search_window_sizeE6size_t

In particular, we should change the following note in the examples:

Note that search_window_size must be at least as large as the desired number of neighbors.
https://intel.github.io/ScalableVectorSearch/python/vamana.html#searching-the-index
https://intel.github.io/ScalableVectorSearch/start.html#searching-the-index
https://intel.github.io/ScalableVectorSearch/start_cpp.html#searching-the-index