Change wording of Requires-Python and offer a lower bound example. Th…

…is should better demonstrate that usage of this field is more commonly about _excluding known incompatible versions_ than it is about specifying _guaranteed compatible versions_. Remove the multiple example code-block because it is not a (multiple use) field, and the other examples seem to exist only to show further usage of PEP 440 version specifiers (which are explained better elsewhere) rather than Requires-Python itself.

source/specifications/core-metadata.rst

@@ -551,18 +551,17 @@ Requires-Python
 .. versionadded:: 1.2
 
 This field specifies the Python version(s) that the distribution is
-guaranteed to be compatible with. Installation tools may look at this when
+compatible with. Installation tools may look at this when
 picking which version of a project to install.
 
 The value must be in the format specified in :doc:`version-specifiers`.
 
-This field cannot be followed by an environment marker.
+For example, if a distribution uses `f-strings <https://docs.python.org/3/whatsnew/3.6.html#pep-498-formatted-string-literals>`_
+then it may prevent installation on Python < 3.6 by specifying:
 
-Examples::
+    Requires-Python: >=3.6
 
-    Requires-Python: >=3
-    Requires-Python: >2.6,!=3.0.*,!=3.1.*
-    Requires-Python: ~=2.6
+This field cannot be followed by an environment marker.
 
 .. _core-metadata-requires-external: