docs: clarify fieldexcludes behavior

[1678092075]

Fixes Or Enhances

Fixes #663.

Clarifies the fieldexcludes documentation to match the current implementation:

• the parameter names another struct field;
• validation checks that the current field's string value does not contain that field's value;
• validation succeeds when the referenced field cannot be resolved;
• the package documentation now uses the registered fieldexcludes tag instead of the nonexistent excludesfield spelling.

No runtime behavior or public API changes.

Verification

• go test -run '^TestFieldExcludes$' .
• make test
• golangci-lint v2.12.2 run
• go doc .

AI assistance disclosure

AI assistance was used to search for a non-duplicated contribution candidate, inspect the implementation and existing tests, draft the documentation wording, and run verification commands.

Affected files: README.md and doc.go.

Human validation required before submission: review the final two-file diff, confirm the wording matches fieldExcludes and TestFieldExcludes, and confirm the commands above passed. The contributor remains responsible for the change and should be able to explain it without AI assistance.

Make sure that you've checked the boxes below before you submit PR:

• Tests exist that cover this behavior (TestFieldExcludes).

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