Use UnreservedCharacter

Author: martinbonnin

spec/Appendix A -- Persisted Documents.md

@@ -107,22 +107,36 @@ PrefixedDocumentIdentifier ::
 - Sha256HexDocumentIdentifier
 - OtherPrefixedDocumentIdentifier
 
-Sha256HexDocumentIdentifier :: `sha256:` Sha256Checksum
-
-Sha256Checksum :: LowerCaseHexDigit+
+Sha256HexDocumentIdentifier :: `sha256:` LowerCaseHexDigit+
 
 LowerCaseHexDigit :: one of
 
 - `0` `1` `2` `3` `4` `5` `6` `7` `8` `9`
 - `a` `b` `c` `d` `e` `f`
 
-OtherPrefixedDocumentIdentifier :: `x-` IdentifierCharacter+ `:` IdentifierCharacter+
+OtherPrefixedDocumentIdentifier :: `x-` UnreservedCharacter+ `:` UnreservedCharacter+
+
+CustomDocumentIdentifier :: UnreservedCharacter+
+
+UnreservedCharacter ::
 
-CustomDocumentIdentifier :: SourceCharacter+
+- Letter
+- Digit
+- `-`
+- `.`
+- `_`
+- `~`
 
-IdentifierCharacter :: SourceCharacter but not `:`
+Letter :: one of
 
-SourceCharacter :: Any Unicode scalar value
+- `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`
+- `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`
+
+Digit :: one of
+
+- `0` `1` `2` `3` `4` `5` `6` `7` `8` `9`
 
 ## Persisting a Document