Add health warning for opaque string comparison

[aphillips]

https://www.w3.org/TR/IndexedDB-2/#object-store-construct

Object store names are always compared in a case-sensitive manner, as opaque sequences of 16-bit code units.

The object store name comparison is fine, so long as "comparison" means "for identity/equality". Case-sensitivity is recommended by the I18N WG and Charmod-Norm. However, a health warning may be warranted here, since it is more than just case-sensitive: it is sensitive to the exact encoding sequence. For example, normalization form affects this. Charmod-Norm provides a somewhat exhaustive list of this kind of variation ;-).

A health warning might be:

Object store names are always compared as opaque sequences of 16-bit code units. As a result, name comparison is sensitive to variations in case as well as other minor variations such as normalization form, the inclusion or omission of controls, and other variations in Unicode text. [[Charmod-Norm]]

This is I18N comment w3c/i18n-activity#363. Please add the i18n-comment label.

[inexorabletash]

Note that databases, object stores, and indexes all have names which behave the same way. I'll spin up a construct for that.

[inexorabletash]

PR: #184

@aphillips - can you review?

(and in case I haven't said it yet - thanks for all the awesome feedback!)