[SPARK-45361][SQL][DOCS] Describe characters unescaping in string lit…

…erals

### What changes were proposed in this pull request?
In the PR, I propose to update the doc ([link](https://spark.apache.org/docs/latest/sql-ref-literals.html#string-literal)) about string literals, and describe unescaping.
<img width="1193" alt="Screenshot 2023-09-28 at 21 23 01" src="https://github.com/apache/spark/assets/1580697/7b871ded-50e1-4c93-9d86-60a2ce93f5e7">

### Why are the changes needed?
To make clear how string literals are preprocessed. This should less confuse Spark SQL users.

### Does this PR introduce _any_ user-facing change?
No.

### How was this patch tested?
By building docs.

### Was this patch authored or co-authored using generative AI tooling?
No.

Closes #43152 from MaxGekk/doc-string-literal.

Authored-by: Max Gekk <max.gekk@gmail.com>
Signed-off-by: Dongjoon Hyun <dhyun@apple.com>

docs/sql-ref-literals.md

@@ -51,6 +51,17 @@ A string literal is used to specify a character string value.
 
     Case insensitive, indicates `RAW`. If a string literal starts with `r` prefix, neither special characters nor unicode characters are escaped by `\`.
 
+The following escape sequences are recognized in regular string literals (without the `r` prefix), and replaced according to the following rules:
+- `\0` -> `\u0000`, unicode character with the code 0;
+- `\b` -> `\u0008`, backspace;
+- `\n` -> `\u000a`, linefeed;
+- `\r` -> `\u000d`, carriage return;
+- `\t` -> `\u0009`, horizontal tab;
+- `\Z` -> `\u001A`, substitute;
+- `\%` -> `\%`;
+- `\_` -> `\_`;
+- `\<other char>` -> `<other char>`, skip the slash and leave the character as is.
+
 #### Examples
 
 ```sql