Clarify grammar for decimal-number cannot have leading zeroes.

src/doc/rustc/src/symbol-mangling/v0.md

@@ -983,12 +983,16 @@ the suffixed name has the same semantics as the original.
 ## Common rules
 [decimal-number]: #common-rules
 [digit]: #common-rules
+[non-zero-digit]: #common-rules
 [lower]: #common-rules
 [upper]: #common-rules
 
-> [decimal-number] → *[digit]* {*[digit]*}
+> [decimal-number] → \
+>       `0` \
+>    | *[non-zero-digit]* {*[digit]*}
 >
-> [digit] → `0` | `1` | `2` | `3` | `4` | `5` | `6` | `7` | `8` | `9`
+> [non-zero-digit] → `1` | `2` | `3` | `4` | `5` | `6` | `7` | `8` | `9` \
+> [digit] → `0` | *[non-zero-digit]*
 >
 > [lower] → `a` |`b` |`c` |`d` |`e` |`f` |`g` |`h` |`i` |`j` |`k` |`l` |`m` |`n` |`o` |`p` |`q` |`r` |`s` |`t` |`u` |`v` |`w` |`x` |`y` |`z`
 >
@@ -1131,11 +1135,14 @@ The following is a summary of all of the productions of the symbol grammar.
 > [vendor-specific-suffix] → (`.` | `$`) *[suffix]* \
 > [suffix] → {*byte*}
 >
-> [decimal-number] → *[digit]* {*[digit]*}
+> [decimal-number] → \
+>       `0` \
+>    | *[non-zero-digit]* {*[digit]*}
 >
 > [base-62-number] → { *[digit]* | *[lower]* | *[upper]* } `_`
 >
-> [digit] → `0` | `1` | `2` | `3` | `4` | `5` | `6` | `7` | `8` | `9` \
+> [non-zero-digit] → `1` | `2` | `3` | `4` | `5` | `6` | `7` | `8` | `9` \
+> [digit] → `0` | *[non-zero-digit]* \
 > [lower] → `a` |`b` |`c` |`d` |`e` |`f` |`g` |`h` |`i` |`j` |`k` |`l` |`m` |`n` |`o` |`p` |`q` |`r` |`s` |`t` |`u` |`v` |`w` |`x` |`y` |`z` \
 > [upper] → `A` | `B` | `C` | `D` | `E` | `F` | `G` | `H` | `I` | `J` | `K` | `L` | `M` | `N` | `O` | `P` | `Q` | `R` | `S` | `T` | `U` | `V` | `W` | `X` | `Y` | `Z`