Add more docstring for the lldb_iter() utility function which provides a compact

way of iterating through an aggregate data structure.  The added example usage
is from an actual use in test/foundation/TestSymbolTable.py:

    2. Pass a container of aggregate which provides APIs to get to the size and
       the element of the aggregate:

    # Module is a container of symbol table 
    module = target.FindModule(filespec)
    for symbol in lldb_iter(module, 'GetNumSymbols', 'GetSymbolAtIndex'):
        name = symbol.GetName()
        ...

llvm-svn: 121271