manifest-schema.yml: say what the validator honours today #137

A documentation correction surfaced by #127 (the "doc nit" in its first verification comment).

manifest-schema.yml line 194, in the header of the schema block, says the validator reads the raw file "honouring delimiter (countries.csv is semicolon-separated) and read_as (positional workbook reads)". builders/_validate.py's read_raw() honours delimiter only; it raises NotImplementedError for any format but csv, and the 13 non-CSV manifests are conformance-only, which the workflow log states per manifest. No CSV manifest carries read_as, so nothing misreads today — the sentence promises a reader that does not exist yet.

To do

Reword the sentence so it describes the validator as shipped: delimiter honoured, parse_dates on every datetime64 column, no index_col; read_as becomes the contract for workbook range reads once those are built (the follow-on #14's closing comment names). One comment line; no field changes.

Exit criterion

Merged on main; scripts/validate_datasets.py still reports 44 pass under both pandas legs of validate-datasets.yml.