Update comments for anything not a valid numeric literal

Author: jskeet

standard/lexical-structure.md

@@ -689,21 +689,21 @@ To permit the smallest possible `int` and `long` values to be written as integer
 > 123                  // decimal, int
 > 10_543_765Lu         // decimal, ulong
 > 1_2__3___4____5      // decimal, int
-> _123                 // invalid; no leading _ allowed
+> _123                 // not a numeric literal; identifier due to leading _
 > 123_                 // invalid; no trailing _allowed
 > 
 > 0xFf                 // hex, int
 > 0X1b_a0_44_fEL       // hex, long
 > 0x1ade_3FE1_29AaUL   // hex, ulong
 > 0x_abc               // hex, int
-> _0x123               // invalid; no leading _ allowed
+> _0x123               // not a numeric literal; identifier due to leading _
 > 0xabc_               // invalid; no trailing _ allowed
 > 
 > 0b101                // binary, int
 > 0B1001_1010u         // binary, uint
 > 0b1111_1111_0000UL   // binary, ulong
 > 0B__111              // binary, int
-> __0B111              // invalid; no leading _ allowed
+> __0B111              // not a numeric literal; identifier due to leading _
 > 0B111__              // invalid; no trailing _ allowed
 > ```
 >
@@ -757,18 +757,18 @@ The value of a real literal of type `float` or `double` is determined by using t
 > *Example*:
 >
 > ```csharp
-> 1.234_567              // double
-> .3e5f                  // float
-> 2_345E-2_0             // double
-> 15D                    // double
-> 19.73M                 // decimal
-> 1.F                    // invalid; ill-formed (parsed as "1." and "F")
-> _1.2F                  // invalid; no leading _ allowed
-> 1_.2F                  // invalid; no trailing _ allowed in integer part
-> 1._234                 // invalid; no leading _ allowed in fraction
-> 1.234_                 // invalid; no trailing _ allowed in fraction
-> .3e_5F                 // invalid; no leading _ allowed in exponent
-> .3e5_F                 // invalid; no trailing _ allowed in exponent
+> 1.234_567      // double
+> .3e5f          // float
+> 2_345E-2_0     // double
+> 15D            // double
+> 19.73M         // decimal
+> 1.F            // parsed as a member access of F due to non-digit after .
+> _1.2F          // invalid; _1 is an identifier due to leading _
+> 1_.2F          // invalid; no trailing _ allowed in integer part
+> 1._234         // parsed as a member access of _234 due to non-digit after .
+> 1.234_         // invalid; no trailing _ allowed in fraction
+> .3e_5F         // invalid; no leading _ allowed in exponent
+> .3e5_F         // invalid; no trailing _ allowed in exponent
 > ```
 >
 > *end example*