Merge #5144

5144: Exactly 512 bytes docid fails r=Kerollmops a=dureuill

# Pull Request

## Related issue
Fixes #5050 

## What does this PR do?
- Return a user error rather than an internal one for docids of exactly 512 bytes
- Fix up error message to indicate that exactly 512 bytes long docids are not supported.
- Fix up error message to reflect that index uids are actually limited to 400 bytes in length

## Impact

- Impacts docs: 
    - update [this paragraph](https://www.meilisearch.com/docs/learn/resources/known_limitations#length-of-primary-key-values) to say 511 bytes instead of 512 

Co-authored-by: Louis Dureuil <louis@meilisearch.com>

crates/milli/src/documents/primary_key.rs

@@ -280,7 +280,7 @@ fn starts_with(selector: &str, key: &str) -> bool {
 
 pub fn validate_document_id_str(document_id: &str) -> Option<&str> {
     if document_id.is_empty()
-        || document_id.len() > 512
+        || document_id.len() >= 512
         || !document_id.chars().all(|c| c.is_ascii_alphanumeric() || c == '-' || c == '_')
     {
         None

crates/milli/src/error.rs

@@ -114,7 +114,7 @@ pub enum UserError {
         "Document identifier `{}` is invalid. \
 A document identifier can be of type integer or string, \
 only composed of alphanumeric characters (a-z A-Z 0-9), hyphens (-) and underscores (_), \
-and can not be more than 512 bytes.", .document_id.to_string()
+and can not be more than 511 bytes.", .document_id.to_string()
     )]
     InvalidDocumentId { document_id: Value },
     #[error("Invalid facet distribution, {}", format_invalid_filter_distribution(.invalid_facets_name, .valid_facets_name))]