The Free Pascal Programmers' Guide is here: https://freepascal.org/docs-html/prog/prog.html#progch1.html
Yes, but the *fix* is in the documentation sources for the FPC 3.2.0 programmers guide and not yet on-line! You have to build the new documentation yourself for now, until 3.2.0 is released.
You can't simply refer to the 3.0.4 documentation. That won't be fixed.
Also note the documentation for 3.0.4. is actually correct, but the "comment" feature is simply omitted.
For documentation online goes the same as for the current release of the compiler itself: the current compiler is 3.0.4 and the current documentation is 3.0.4.
In the wiki that I will finish sometime today, I will make the point that this feature has always been there, at least since fpc 1.9
Stay put. I'll let you know when I am finished with the rewrite.
In the mean time try to build the new programmers guide from source...
(warning: that is not easy the first time you try!)
Or read the raw TeX...