PostgreSQL 15 minor fixes in protocol.sgml

Hello!

I've come across some typos in protocol.sgml for PostgreSQL 15 so please 
have a look at the attached patch.

I didn't include it in the patch but I also suggest removing single 
quotes around 'method' for the COMPRESSION option to help avoid 
confusion. (All the supported compression methods consist of a single 
word so in my opinion there is no need to use quotes in this case.)
-- <term><literal>COMPRESSION</literal> 
<replaceable>'method'</replaceable></term>

I've also noticed that there are two ways to describe an option: "If set 
to true" / "If true". As far as I know, the option here is specified by 
its name rather than being explicitly set to true so "if true" seems to 
be more correct, and this could be a slight improvement for this page. 
Please correct me if I'm wrong.

Another point worth mentioning is that only this file contains the 
phrase "two-phase transaction". I believe that "two-phase commit 
transaction" or "transaction prepared for two-phase commit" depending on 
the situation would be better wording.

And finally, could you please clarify this part?
-- The end LSN of the prepare transaction.
Is it a typo of "prepared transaction"? Or is it the LSN of the 
transaction for Prepare?
If it's the latter, perhaps it'd make more sense to capitalize it.

-- 
Best regards,
Ekaterina Kiryanova
Technical Writer
Postgres Professional
the Russian PostgreSQL Company