Improve documentation of `vector_embedding_error`

[guimachiavelli]

From this Slack discussion (private link):

Meilisearch is complaining that:

• The user is using a userProvided embedder
• The user is not correctly providing vectors for this embedder for at least one document.

They should get the document described by the error message, and check:

• it has a _vectors field
• a subfield of the _vectors field is named pixolutionV5
• the value of that subfield is one of:
  • null, to opt-out of embedding for this document
  • [...] an array of 128 numbers
  • [[...], ..., [...]] an array of arrays of 128 numbers

To fix the error:

The correct procedure would be:

• Fix the _vectors field in all documents to contain the embedders following the syntax described above
• Attempt to add the embedder again

Tasks

• Improve reference/errors/error_codes#vector_embedding_error by adding more information of what might have gone wrong, as described in the quotes above