SDL_strlen and SDL_strnlen referenced in docs but undocumented

[DyXel]

Some pages such as https://wiki.libsdl.org/SDL3/SDL_strlcat reference it, but lead to a "no such page".

[icculus]

Yeah, our biggest documentation coverage gaps right now are these C runtime wrappers. I'll try to take a run at these for 3.2.0, even if just to say something like "this works exactly like strlen() but doesn't require access to a C runtime."