Kotlin: Mention Literal::getLiteral() difference from source code

It appears the Kotlin extractor does not have access to the actual
string representation in the source code, and for most literal types
uses simply the represented value also as `getLiteral` result, see
https://github.com/github/codeql/blob/codeql-cli/v2.15.1/java/kotlin-extractor/src/main/kotlin/KotlinFileExtractor.kt#L4443

Author: Marcono1234

java/ql/lib/semmle/code/java/Expr.qll

@@ -530,6 +530,9 @@ class Literal extends Expr, @literal {
    * Gets a string representation of this literal as it appeared
    * in the source code.
    *
+   * For Kotlin the result might not match the exact representation
+   * used in the source code.
+   *
    * **Important:** Unless a query explicitly wants to check how
    * a literal was written in the source code, the predicate
    * `getValue()` (or value predicates of subclasses) should be