Update docs to clarify that id is not a required field for cache.evict(options)

[aayush-k]

Tried this without looking at which options were required/not required and realized that just providing a fieldName is sufficient when i want to evict all cached entries for a specific gql query. I also found that @benjamn mentioned the same solution/implied that id is optional in #6795 (comment)

The sourcecode also indicates that the default default value is ROOT_QUERY https://github.com/apollographql/apollo-client/blob/main/src/cache/inmemory/inMemoryCache.ts#L266-L272

Checklist:

• If this PR is a new feature, please reference an issue where a consensus about the design was reached (not necessary for small changes) (N/A)
• Make sure all of the significant new logic is covered by tests (N/A)

[apollo-cla]

@aayush-k: Thank you for submitting a pull request! Before we can merge it, you'll need to sign the Apollo Contributor License Agreement here: https://contribute.apollographql.com/

[aayush-k]

Also I stumbled across this comment:

apollo-client/src/cache/inmemory/inMemoryCache.ts

Lines 161 to 169 in dfd159e

	// To my knowledge, TypeScript does not currently provide a way to
	// enforce that an optional property?:type must *not* be undefined
	// when present. That ability would be useful here, because we want
	// options.id to default to ROOT_QUERY only when no options.id was
	// provided. If the caller attempts to pass options.id with a
	// falsy/undefined value (perhaps because cache.identify failed), we
	// should not assume the goal was to modify the ROOT_QUERY object.
	// We could throw, but it seems natural to return false to indicate
	// that nothing was modified.

I'm not sure if this was the reason why id was originally labeled as "Required" in the docs but maybe this would help:

const modify = (x: ({ id: string; optimistic?: boolean } | {optimistic?: boolean})) => x;
modify({ id: 'string' }); // <- ok
modify({ id: undefined }); // <- errors
modify({}); // <- ok

[StephenBarlow]

Thanks @aayush-k! I believe this was incorrectly marked as required purely because I incorrectly thought it was required 😄