Try to completely remove the word symbol and use nonterminal

The ones that we could not remove are in grammar.py, because that's the public documented API.

parso/grammar.py

@@ -51,8 +51,8 @@ class Grammar(object):
             it is invalid, it will be returned as an error node. If disabled,
             you will get a ParseError when encountering syntax errors in your
             code.
-        :param str start_symbol: The grammar symbol that you want to parse. Only
-            allowed to be used when error_recovery is False.
+        :param str start_symbol: The grammar rule (nonterminal) that you want
+            to parse. Only allowed to be used when error_recovery is False.
         :param str path: The path to the file you want to open. Only needed for caching.
         :param bool cache: Keeps a copy of the parser tree in RAM and on disk
             if a path is given. Returns the cached trees if the corresponding
@@ -88,7 +88,7 @@ class Grammar(object):
             raise TypeError("Please provide either code or a path.")
 
         if start_symbol is None:
-            start_symbol = self._start_symbol
+            start_symbol = self._start_nonterminal
 
         if error_recovery and start_symbol != 'file_input':
             raise NotImplementedError("This is currently not implemented.")
@@ -136,7 +136,7 @@ class Grammar(object):
         p = self._parser(
             self._pgen_grammar,
             error_recovery=error_recovery,
-            start_symbol=start_symbol
+            start_nonterminal=start_symbol
         )
         root_node = p.parse(tokens=tokens)
 
@@ -186,7 +186,7 @@ class Grammar(object):
         return normalizer.issues
 
     def __repr__(self):
-        labels = self._pgen_grammar.number2symbol.values()
+        labels = self._pgen_grammar.number2nonterminal.values()
         txt = ' '.join(list(labels)[:3]) + ' ...'
         return '<%s:%s>' % (self.__class__.__name__, txt)
 
@@ -194,7 +194,7 @@ class Grammar(object):
 class PythonGrammar(Grammar):
     _error_normalizer_config = ErrorFinderConfig()
     _token_namespace = token
-    _start_symbol = 'file_input'
+    _start_nonterminal = 'file_input'
 
     def __init__(self, version_info, bnf_text):
         super(PythonGrammar, self).__init__(