bpo-12910: update and correct quote docstring (#2568)

Fixes some mistakes and misleadings in the quote function docstring:
- reserved chars are never actually used by quote code, unreserved chars are
- reserved chars were wrong and incomplete
- mentioned that use-case is not minimal quoting wrt. RFC, but cautious quoting

Lib/urllib/parse.py

@@ -785,25 +785,32 @@ def quote(string, safe='/', encoding=None, errors=None):
     """quote('abc def') -> 'abc%20def'
 
     Each part of a URL, e.g. the path info, the query, etc., has a
-    different set of reserved characters that must be quoted.
+    different set of reserved characters that must be quoted. The
+    quote function offers a cautious (not minimal) way to quote a
+    string for most of these parts.
 
-    RFC 3986 Uniform Resource Identifiers (URI): Generic Syntax lists
-    the following reserved characters.
+    RFC 3986 Uniform Resource Identifier (URI): Generic Syntax lists
+    the following (un)reserved characters.
 
-    reserved    = ";" | "/" | "?" | ":" | "@" | "&" | "=" | "+" |
-                  "$" | "," | "~"
+    unreserved    = ALPHA / DIGIT / "-" / "." / "_" / "~"
+    reserved      = gen-delims / sub-delims
+    gen-delims    = ":" / "/" / "?" / "#" / "[" / "]" / "@"
+    sub-delims    = "!" / "$" / "&" / "'" / "(" / ")"
+                  / "*" / "+" / "," / ";" / "="
 
-    Each of these characters is reserved in some component of a URL,
+    Each of the reserved characters is reserved in some component of a URL,
     but not necessarily in all of them.
 
-    Python 3.7 updates from using RFC 2396 to RFC 3986 to quote URL strings.
-    Now, "~" is included in the set of reserved characters.
+    The quote function %-escapes all characters that are neither in the
+    unreserved chars ("always safe") nor the additional chars set via the
+    safe arg.
+
+    The default for the safe arg is '/'. The character is reserved, but in
+    typical usage the quote function is being called on a path where the
+    existing slash characters are to be preserved.
 
-    By default, the quote function is intended for quoting the path
-    section of a URL.  Thus, it will not encode '/'.  This character
-    is reserved, but in typical usage the quote function is being
-    called on a path where the existing slash characters are used as
-    reserved characters.
+    Python 3.7 updates from using RFC 2396 to RFC 3986 to quote URL strings.
+    Now, "~" is included in the set of unreserved characters.
 
     string and safe may be either str or bytes objects. encoding and errors
     must not be specified if string is a bytes object.