Rollup merge of #70418 - PankajChaudhary5:master, r=Dylan-DPC

Add long error explanation for E0703

Add long explanation for the E0703 error code
Part of #61137

r? @GuillaumeGomez

src/librustc_error_codes/error_codes/E0703.md

@@ -0,0 +1,17 @@
+Invalid ABI (Application Binary Interface) used in the code.
+
+Erroneous code example:
+
+```compile_fail,E0703
+extern "invalid" fn foo() {} // error!
+# fn main() {}
+```
+
+At present few predefined ABI's (like Rust, C, system, etc.) can be
+used in Rust. Verify that the ABI is predefined. For example you can
+replace the given ABI from 'Rust'.
+
+```
+extern "Rust" fn foo() {} // ok!
+# fn main() { }
+```