regparam flag description improved

Co-authored-by: Ralf Jung <post@ralfj.de>

Author: azhogin

compiler/rustc_session/src/options.rs

@@ -1980,7 +1980,8 @@ options! {
         "randomize the layout of types (default: no)"),
     regparm: Option<u32> = (None, parse_opt_number, [TRACKED],
         "On x86-32 targets, the regparm N causes the compiler to pass N arguments \
-        in registers EAX, EDX, and ECX instead of on the stack."),
+        in registers EAX, EDX, and ECX instead of on the stack.\
+        It is UNSOUND to link together crates that use different values for this flag!"),
     relax_elf_relocations: Option<bool> = (None, parse_opt_bool, [TRACKED],
         "whether ELF relocations can be relaxed"),
     remap_cwd_prefix: Option<PathBuf> = (None, parse_opt_pathbuf, [TRACKED],