Document: don't allow characters with unicode property Bidi_Class in …

…source files

Update documentation with changes made in scala#10017, scala#10023, scala#10030

spec/01-lexical-syntax.md

@@ -42,6 +42,7 @@ classes (Unicode general category given in parentheses):
 1. Operator characters. These consist of all printable ASCII characters
    (`\u0020` - `\u007E`) that are in none of the sets above, mathematical
    symbols (`Sm`) and other symbols (`So`).
+1. [Bidirectional explicit formatting](https://www.unicode.org/reports/tr9/#Bidirectional_Character_Types) characters. The nine characters `\u202a - \u202e` and `\u2066 - \u2069`, inclusive. These are forbidden from appearing in character and string literals; in Scala 2.12 they may not appear in source files at all.
 
 ## Identifiers
 
@@ -403,12 +404,12 @@ members of type `Boolean`.
 ### Character Literals
 
 ```ebnf
-characterLiteral  ::=  ‘'’ (charNoQuoteOrNewline | UnicodeEscape | charEscapeSeq) ‘'’
+characterLiteral  ::=  ‘'’ (charNoQuoteOrNewlineOrBidiFormatting | UnicodeEscape | charEscapeSeq) ‘'’
 ```
 
 A character literal is a single character enclosed in quotes.
 The character can be any Unicode character except the single quote
-delimiter or `\u000A` (LF) or `\u000D` (CR);
+delimiter or `\u000A` (LF) or `\u000D` (CR) or a bidirectional formatting character;
 or any Unicode character represented by either a
 [Unicode escape](01-lexical-syntax.html) or by an [escape sequence](#escape-sequences).
 
@@ -426,12 +427,12 @@ which can also be written using the escape sequence `'\n'`.
 
 ```ebnf
 stringLiteral  ::=  ‘"’ {stringElement} ‘"’
-stringElement  ::=  charNoDoubleQuoteOrNewline | UnicodeEscape | charEscapeSeq
+stringElement  ::=  charNoDoubleQuoteOrNewlineOrBidiFormatting | UnicodeEscape | charEscapeSeq
 ```
 
 A string literal is a sequence of characters in double quotes.
 The characters can be any Unicode character except the double quote
-delimiter or `\u000A` (LF) or `\u000D` (CR);
+delimiter or `\u000A` (LF) or `\u000D` (CR) or a bidirectional formatting character;
 or any Unicode character represented by either a
 [Unicode escape](01-lexical-syntax.html) or by an [escape sequence](#escape-sequences).
 
@@ -449,13 +450,13 @@ The value of a string literal is an instance of class `String`.
 
 ```ebnf
 stringLiteral   ::=  ‘"""’ multiLineChars ‘"""’
-multiLineChars  ::=  {[‘"’] [‘"’] charNoDoubleQuote} {‘"’}
+multiLineChars  ::=  {[‘"’] [‘"’] charNoDoubleQuoteOrBidiFormatting} {‘"’}
 ```
 
 A multi-line string literal is a sequence of characters enclosed in
 triple quotes `""" ... """`. The sequence of characters is
 arbitrary, except that it may contain three or more consecutive quote characters
-only at the very end. Characters
+only at the very end; the only forbidden Unicode characters are the bidirectional formatting ones. Characters
 must not necessarily be printable; newlines or other
 control characters are also permitted.  Unicode escapes work as everywhere else, but none
 of the escape sequences [here](#escape-sequences) are interpreted.

spec/13-syntax-summary.md

@@ -31,6 +31,7 @@ opchar           ::= // printableChar not matched by (whiteSpace | upper | lower
                      // letter | digit | paren | delim | opchar | Unicode_Sm | Unicode_So)
 printableChar    ::= // all characters in [\u0020, \u007F] inclusive
 charEscapeSeq    ::= ‘\’ (‘b’ | ‘t’ | ‘n’ | ‘f’ | ‘r’ | ‘"’ | ‘'’ | ‘\’)
+bidiFormatting   ::= // all characters in [\u202a, \u202e] and [\u2066, \u2069], inclusive
 
 op               ::=  opchar {opchar}
 varid            ::=  lower idrest
@@ -57,14 +58,14 @@ floatType        ::=  ‘F’ | ‘f’ | ‘D’ | ‘d’
 
 booleanLiteral   ::=  ‘true’ | ‘false’
 
-characterLiteral ::=  ‘'’ (charNoQuoteOrNewline | UnicodeEscape | charEscapeSeq) ‘'’
+characterLiteral ::=  ‘'’ (charNoQuoteOrNewlineOrBidiFormatting | UnicodeEscape | charEscapeSeq) ‘'’
 
 stringLiteral    ::=  ‘"’ {stringElement} ‘"’
                  |  ‘"""’ multiLineChars ‘"""’
-stringElement    ::=  charNoDoubleQuoteOrNewline
+stringElement    ::=  charNoDoubleQuoteOrNewlineOrBidiFormatting
                  |  UnicodeEscape
                  |  charEscapeSeq
-multiLineChars   ::=  {[‘"’] [‘"’] charNoDoubleQuote} {‘"’}
+multiLineChars   ::=  {[‘"’] [‘"’] charNoDoubleQuoteOrBidiFormatting} {‘"’}
 
 symbolLiteral    ::=  ‘'’ plainid