Clarify behavior of advance(..., skip) in external scanner docs

Clarifies the behavior of `TSLexer::advance` when `skip=true`.

The previous documentation didn't make it clear that `skip=true` should only be used before a token starts. Using it after `mark_end` can affect the token’s starting position and lead to incorrect or zero-length ranges.

Fixes #2315

docs/src/creating-parsers/4-external-scanners.md

@@ -138,9 +138,11 @@ The second parameter to this function is the lexer, of type `TSLexer`. The `TSLe
 - **`TSSymbol result_symbol`** — The symbol that was recognized. Your scan function should _assign_ to this field one of
 the values from the `TokenType` enum, described above.
 
-- **`void (*advance)(TSLexer *, bool skip)`** — A function for advancing to the next character. If you pass `true` for
-the second argument, the current character will be treated as whitespace; whitespace won't be included in the text range
-associated with tokens emitted by the external scanner.
+- **`void (*advance)(TSLexer *, bool skip)`** — A function for advancing to the next character. If `skip` is `true`, the
+current character will be treated as skipped text and will not be included in the text range associated with tokens emitted
+by the external scanner. This is typically used before a token starts, such as when skipping leading whitespace. After a
+token has started, `skip` should generally be `false`. In particular, calling `advance(lexer, true)` after `mark_end` can
+affect the token's starting position and lead to incorrect or zero-length token ranges.
 
 - **`void (*mark_end)(TSLexer *)`** — A function for marking the end of the recognized token. This allows matching tokens
 that require multiple characters of lookahead. By default, (if you don't call `mark_end`), any character that you moved