Limited -Zregparm support (no Rust calling conv) descriptions

Co-authored-by: Jubilee <[email protected]>

Author: azhogin

Diff for: compiler/rustc_session/src/options.rs

@@ -2002,7 +2002,8 @@ options! {
         "randomize the layout of types (default: no)"),
     regparm: Option<u32> = (None, parse_opt_number, [TRACKED],
         "On x86-32 targets, setting this to 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 for\
+        \"C\", \"cdecl\", and \"stdcall\" fn.\
         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"),

Diff for: src/doc/unstable-book/src/compiler-flags/regparm.md

@@ -5,7 +5,7 @@ The tracking issue for this feature is: https://github.com/rust-lang/rust/issues
 ------------------------
 
 Option -Zregparm=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 for "C", "cdecl", and "stdcall" fn.
 It is UNSOUND to link together crates that use different values for this flag.
 It is only supported on `x86`.