docs: note vf() view filters are exact-match only

[jacalata]

Adds a note near the shared ImageRequestOptions / PDFRequestOptions / CSVRequestOptions section explaining that vf() view filters are exact-match only, with a pointer to the Tableau REST API docs and mentioning the workbook-parameter workaround for range filtering.

Followup to closed PR #1813, which had tried to add operator support (gt:, lt: etc.) to vf() client-side — verified against server code that the vf_* REST endpoint parses values by data type with no operator recognition, silently dropping filters like vf_sales=gt:1000. The client can't fix this; a note is the right response.

Test plan

• visual review of the rendered section — sits between the "You can use ImageRequestOptions and PDFRequestOptions..." intro and the "For more information about filtering and sorting..." link
• once merged, links resolve on tableau.github.io/server-client-python