Officially, there are no public documents describing the error and warning messages. The messages are supposed to be self-explanatory in context. Of course, the "self-explanatory" part might assume more expertise in the details of the language than the programmer has, and sometimes a message is too cryptic. (Sometimes a message is misleading, but no document would help with that.)
As it happens, there is an html file containing all the messages for the compiler, along with brief explanations and some examples. This file was created for use with an old IDE that we no longer provide, and a future IDE might start using these files again. The files for C and C++ are generally kept up to date, but the one for Fortan might not always be current.
See directory <INSTALL_DIR>/prod/lib/locale/C/LC_MESSAGES
You can bring up the file in a browser and search for messages in question. Hint: Compile with the option -errtags to see the unique tag for each message, and search for the tag in the html file.
Caveats: These files were not intended to be publicly accessible. The file names and location could change without notice, or disappear entirely. The contents are not necessarily complete or correct.