ACTION-2274: Suggest function grouping

All,

I suggest grouping functions by area rather than return type.

I also added some annotations:

- [XF11]: present since XForms 1.1 (or 1.0)
- [XF20]: present since XForms 2.0
- [CLASHES]: XPath 2.0 has a function with the same name that clashes. We
make a recommendation in the spec.

Here is the proposed grouping (groups are in no particular order).

## Utility functions

4.6.2 The current() Function [XF11]
4.6.4 The context() Function [XF11]

4.2.1 The boolean-from-string() Function [XF11]
4.3.4 The count-non-empty() Function [XF11]

4.8.3 The eval() Function [XF20]
4.8.4 The eval-in-context() Function [XF20]

## Math functions

4.3.1 The avg() Function [XF11][CLASHES]
4.3.2 The min() Function [XF11][CLASHES]
4.3.3 The max() Function [XF11][CLASHES]
4.3.6 The power() Function [XF11][NOTE: XPath 3 has `pow()``]
4.3.7 The random() Function [XF11]

## Cryptographic functions

4.4.3 The digest() Function [XF11]
4.4.4 The hmac() Function [XF11]

## Domain functions

4.2.2 The is-card-number() Function [XF11]

## Model functions

4.6.1 The instance() Function [XF11]
4.6.5 The bind() Function [XF20]
4.2.3 The valid() Function [XF20]
4.2.4 The relevant() Function [XF20]
4.2.5 The readonly() Function [XF20]
4.2.6 The required() Function [XF20]

## Controls functions

4.3.5 The index() Function [XF11]
4.4.1 The case() Function [XF20]

## Other XForms functions

4.8.2 The event() Function [XF11]
4.4.2 The property() Function [XF11]

## 4.5 Date and time functions

4.5.1 The local-date() Function [XF11]
4.5.2 The local-dateTime() Function [XF11]
4.5.3 The now() Function [XF11]
4.5.4 The days-from-date() Function [XF11]
4.5.5 The days-to-date() Function [XF11]
4.5.6 The seconds-from-epoch() Function [XF20]
4.5.7 The seconds-from-dateTime() Function [XF11][CLASHES]
4.5.8 The seconds-to-dateTime() Function [XF11]
4.5.9 The adjust-dateTime-to-timezone() Function [XF11][CLASHES]
4.5.10 The seconds() Function [XF11]
4.5.11 The months() Function [XF11]

## 4.9 URI functions

4.9.1 The location-uri() Function [XF20]
4.9.2 The location-param() Function [XF20]
4.9.3 The uri-scheme() Function [XF20]
4.9.4 The uri-scheme-specific-part() Function [XF20]
4.9.5 The uri-authority() Function [XF20]
4.9.6 The uri-user-info() Function [XF20]
4.9.7 The uri-host() Function [XF20]
4.9.8 The uri-port() Function [XF20]
4.9.9 The uri-path() Function [XF20]
4.9.10 The uri-query() Function [XF20]
4.9.11 The uri-fragment() Function [XF20]
4.9.12 The uri-param-names() Function [XF20]
4.9.13 The uri-param-values() Function [XF20]

## XML manipulation functions

4.4.5 The serialize() Function [XF20][NOTE: same as XPath 3.0]
4.7.1 The element() Function [XF20]
4.7.2 The attribute() Function [XF20]
4.7.3 The parse() Function [XF20]

## String functions

4.3.8 The compare() Function [XF11][TODO: add note about XPath 2.0]

## Deprecated functions

4.8.1 The choose() Function [XF11][TODO: deprecate?]
4.6.3 The id() Function [XF11][DEPRECATED]

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