docs: Give the string input conversion function in the string tutorial a...

docs: Give the string input conversion function in the string tutorial a better name and clarify some comments.

docs/examples/tutorial/string/api_func.pyx

-from to_unicode cimport _ustring
+from to_unicode cimport _text
 
 def api_func(s):
-    text = _ustring(s)
+    text_input = _text(s)
     # ...

docs/examples/tutorial/string/to_unicode.pxd

-cdef unicode _ustring(s)
+cdef unicode _text(s)

docs/examples/tutorial/string/to_unicode.pyx

@@ -2,19 +2,20 @@
 
 from cpython.version cimport PY_MAJOR_VERSION
 
-cdef unicode _ustring(s):
+cdef unicode _text(s):
     if type(s) is unicode:
-        # fast path for most common case(s)
+        # Fast path for most common case(s).
         return <unicode>s
 
     elif PY_MAJOR_VERSION < 3 and isinstance(s, bytes):
-        # only accept byte strings in Python 2.x, not in Py3
+        # Only accept byte strings as text input in Python 2.x, not in Py3.
         return (<bytes>s).decode('ascii')
 
     elif isinstance(s, unicode):
-        # an evil cast to <unicode> might work here in some(!) cases,
-        # depending on what the further processing does.  to be safe,
-        # we can always create a copy instead
+        # We know from the fast path above that 's' can only be a subtype here.
+        # An evil cast to <unicode> might still work in some(!) cases,
+        # depending on what the further processing does.  To be safe,
+        # we can always create a copy instead.
         return unicode(s)
 
     else: