Do not use C++ style comments (// comments). Strict ANSI C compilers do not accept them.

The following documentation comment has been logged on the website:

Page: https://www.postgresql.org/docs/11/source-format.html
Description:

This sentence in the style guide feels outdated. // style comments have been
in the ANSI C standard for almost 20 years now (C99 was accepted as ANSI
standard in may 2000). I suggest rewording this to "Do not use C99 style
comments (// comments). Strict C89 compilers do not accept them." or come up
with some other reason for /**/ style being preferred to // style since I'm
not entierly convinced postgres compiles on C89 anyways.