[PGdocs] fix description for handling pf non-ASCII characters

Dear hackers,

While discussing based on the article[1] with Japanese developers,
I found inconsistencies between codes and documents.

45b1a67a[2] changed the behavior when non-ASCII characters was set as application_name,
cluster_name and postgres_fdw.application_name, but it seemed not to be documented.
Previously non-ASCII chars were replaed with question makrs '?', but now they are replaced
with a hex escape instead.

How do you think? Is my understanding correct?

Acknowledgement:
Sawada-san and Shinoda-san led the developer's discussion.
Fujii-san was confirmed my points. Thank you for all of their works!

[1]:
https://h50146.www5.hpe.com/products/software/oe/linux/mainstream/support/lcc/pdf/PostgreSQL16Beta1_New_Features_en_20230528_1.pdf
[2]:
https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=45b1a67a0fcb3f1588df596431871de4c93cb76f;hp=da5d4ea5aaac4fc02f2e2aec272efe438dd4e171

Best Regards,
Hayato Kuroda
FUJITSU LIMITED