tweak discriminant on non-nullary enum diagnostic

Adds notes pointing at the non-nullary variants, and uses "custom
discriminant" language to be consistent with the Reference.

src/test/ui/parser/issue-17383.rs

@@ -1,6 +1,6 @@
 enum X {
     A = 3,
-    //~^ ERROR discriminator values can only be used with a field-less enum
+    //~^ ERROR custom discriminant values are not allowed in enums with fields
     B(usize)
 }
 

src/test/ui/parser/issue-17383.stderr

@@ -1,8 +1,11 @@
-error: discriminator values can only be used with a field-less enum
+error: custom discriminant values are not allowed in enums with fields
   --> $DIR/issue-17383.rs:2:9
    |
 LL |     A = 3,
-   |         ^ only valid in field-less enums
+   |         ^ invalid custom discriminant
+LL |
+LL |     B(usize)
+   |     -------- variant with a field defined here
 
 error: aborting due to previous error
 

src/test/ui/parser/tag-variant-disr-non-nullary.rs

@@ -1,11 +1,12 @@
 enum Color {
     Red = 0xff0000,
-    //~^ ERROR discriminator values can only be used with a field-less enum
+    //~^ ERROR custom discriminant values are not allowed in enums with fields
     Green = 0x00ff00,
     Blue = 0x0000ff,
     Black = 0x000000,
     White = 0xffffff,
     Other(usize),
+    Other2(usize, usize),
 }
 
 fn main() {}

src/test/ui/parser/tag-variant-disr-non-nullary.stderr

@@ -1,17 +1,21 @@
-error: discriminator values can only be used with a field-less enum
+error: custom discriminant values are not allowed in enums with fields
   --> $DIR/tag-variant-disr-non-nullary.rs:2:11
    |
 LL |     Red = 0xff0000,
-   |           ^^^^^^^^ only valid in field-less enums
+   |           ^^^^^^^^ invalid custom discriminant
 LL |
 LL |     Green = 0x00ff00,
-   |             ^^^^^^^^ only valid in field-less enums
+   |             ^^^^^^^^ invalid custom discriminant
 LL |     Blue = 0x0000ff,
-   |            ^^^^^^^^ only valid in field-less enums
+   |            ^^^^^^^^ invalid custom discriminant
 LL |     Black = 0x000000,
-   |             ^^^^^^^^ only valid in field-less enums
+   |             ^^^^^^^^ invalid custom discriminant
 LL |     White = 0xffffff,
-   |             ^^^^^^^^ only valid in field-less enums
+   |             ^^^^^^^^ invalid custom discriminant
+LL |     Other(usize),
+   |     ------------ variant with a field defined here
+LL |     Other2(usize, usize),
+   |     -------------------- variant with fields defined here
 
 error: aborting due to previous error