Document that for "usual" regex behavior multiline is required

Regular expression users typically expect that matching a `$` in a
multiline string would match the end of current line and not the end
of the string past many lines. This is default behavior in pretty much
every regexp engine: `grep`, `perl`, text editors, you name it… So it
is fair to expect such expectation, so warn a user about necessity to
pass `multiline`

Fixes: purescript-contrib#231

Author: Hi-Angel

Diff for: src/Parsing/String.purs

@@ -195,6 +195,9 @@ match p = do
 
 -- | Compile a regular expression `String` into a regular expression parser.
 -- |
+-- | Note that per JS RegExp semantics matching a single line in a multiline
+-- | string requires passing `multiline` flag rather than `noFlags`.
+-- |
 -- | This function will use the `Data.String.Regex.regex` function to compile
 -- | and return a parser which can be used
 -- | in a `ParserT String m` monad.
@@ -235,7 +238,7 @@ match p = do
 -- | capture the regular expression pattern `x*`.
 -- |
 -- | ```purescript
--- | case regex "x*" noFlags of
+-- | case regex "x*" multiline of
 -- |   Left compileError -> unsafeCrashWith $ "xMany failed to compile: " <> compileError
 -- |   Right xMany -> runParser "xxxZ" do
 -- |     xMany