docs: python bindings - protonmail examples

* lang/python/docs/GPGMEpythonHOWTOen.org: Updated links to the
  ProtonMail keyserver import scripts and added a warning regarding
  being unable to update third party keys.
* lang/python/examples/howto/pmkey-import-alt.py: added usage.
* lang/python/examples/howto/pmkey-import.py: added usage.

lang/python/docs/GPGMEpythonHOWTOen.org

@@ -383,7 +383,7 @@ There are two theoretical solutions to this issue:
  2. Compile and install Python using the same tools used by choice,
     such as MinGW or Msys2.
 
-Do *NOT* use the official Windows installer for Python unless
+Do *not* use the official Windows installer for Python unless
 following the first method.
 
 In this type of situation it may even be for the best to accept that
@@ -785,6 +785,18 @@ The key IDs for all considered keys were:
         print(e)
 #+END_SRC
 
+Both the above example, [[../examples/howto/pmkey-import.py][pmkey-import.py]], and a version which prompts
+for an alternative GnuPG home directory, [[../examples/howto/pmkey-import-alt.py][pmkey-import-alt.py]], are
+available with the other examples and are executable scripts.
+
+Note that while the ProtonMail servers are based on the SKS servers,
+their server is related more to their API and is not feature complete
+by comparison to the servers in the SKS pool.  One notable difference
+being that the ProtonMail server does not permit non ProtonMail users
+to update their own keys, which could be a vector for attacking
+ProtonMail users who may not receive a key's revocation if it had been
+compromised.
+
 
 ** Exporting keys
    :PROPERTIES:

lang/python/examples/howto/pmkey-import-alt.py

@@ -34,6 +34,8 @@ del absolute_import, division, unicode_literals
 print("""
 This script searches the ProtonMail key server for the specified key and
 imports it.  Optionally enables specifying a different GnuPG home directory.
+
+Usage:  pmkey-import-alt.py [search string] [homedir]
 """)
 
 c = gpg.Context(armor=True)

lang/python/examples/howto/pmkey-import.py

@@ -33,6 +33,8 @@ del absolute_import, division, unicode_literals
 print("""
 This script searches the ProtonMail key server for the specified key and
 imports it.
+
+Usage:  pmkey-import.py [search string]
 """)
 
 c = gpg.Context(armor=True)