docs: expand update_connection_auth sample header for Snowflake keypair

[jacalata]

Closes #1602.

Motivation

#1602 asked for an end-to-end example of converting a Tableau Cloud
datasource or workbook connection from Snowflake username/password auth
to keypair auth. The existing update_connection_auth.py already runs
the right code path -- it takes authentication_type as an arg and
sets embed_password=True. What it was missing was context: which
authentication_type strings are common, and the pre-saved-credential
prerequisite that applies to any embed_password conversion.

Initial draft of this PR added a dedicated keypair sample; folded that
into a header-comment update on update_connection_auth.py after
noticing the code duplication. See 949d602 for the fold-in.

Behavior change

Samples only -- no library changes. Adds a ~25-line header comment to
samples/update_connection_auth.py:

• Lists common authentication_type values including auth-keypair,
  oauth, AD Service Principal, sqlserver, and Username Password
• Explains that embed_password=True binds the connection to a
  matching pre-saved credential on the site, looked up by
  (username, connection class, role)
• Spells out the Snowflake-keypair prerequisite: the private key must
  be saved on the site under Site Settings -> Saved Credentials for
  Data Sources before the sample runs. Without it, the update writes
  the new auth type into metadata but subsequent extract refreshes
  and connection tests fail.
• Notes REST API version support (v3.27 for datasource/workbook
  connections, v3.28 for flows)

Test plan

Docs-only change to a sample; no automated tests exist for samples/.

• python samples/update_connection_auth.py --help still shows the
  same args (help output unchanged)
• Manual: against a Tableau Cloud site with a pre-saved Snowflake
  private-key credential, run the sample with
  authentication_type=auth-keypair; connection's auth_type flips
  and extract refresh succeeds
• Manual: without pre-saved credentials, connection test fails
  post-update as documented

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