Update Embeddings - Use update instead of replace

[venuraja79]

Proposed changes:

• Weaviate's replace method replaces the complete document with a new one where as "update" only updates the properties that are passed to the method and rest of the properties will be in tact.
• Due to a defect in v1.3.0, Update Embeddings was implemented with replace. Now, changing the logic to use "update".

This is a minor change, all tests are passing.

Status (please check what you already did):

• First draft (up for discussions & feedback)
• Final code
• Added tests
• Updated documentation

[venuraja79]

Linking #1159
@tholor, @lalitpagaria - please review and take it forward.

[tholor]

Thanks for the fix!