Add information about multipart/form-data in api-client

[mkucmus]

Describe the feature

the proper information must be displayed once content-type: multipart/form-data is being used in api-client (especially the client side)

Description

the problem was reported in the community slack https://shopwarecommunity.slack.com/archives/C050L6NCMGQ/p1742555220417229

Acceptance Criteria

• api-client is updated
• the exception is mentioned in the docs

Additional information

• Would you be willing to help implement this feature?

Checks

• Check existing discussions and issues.