[SMTChecker]: document currently unsupported features

## Page

[SMTChecker](https://docs.soliditylang.org/en/v0.8.18/smtchecker.html).

## Issue Description

I've recently started using the SMTChecker on a code base that uses many of the modern Solidity features, and I was disappointed to see that many features are not yet supported. To share just a few examples:

```text
warning[7650]: Warning: Assertion checker does not yet support this expression.
--> src/SablierV2LockupLinear.sol:370:17:
|
370 | if (recipient.code.length > 0) {
| ^^^^^^^^^^^^^^
```

The above is the [`.code` member of the `address` type](https://docs.soliditylang.org/en/v0.8.18/units-and-global-variables.html#members-of-address-types).

Then:

```text
warning[8364]: Warning: Assertion checker does not yet implement type type(struct LockupLinear.Stream storage pointer)
--> src/SablierV2LockupLinear.sol:419:30:
|
419 | _streams[streamId] = LockupLinear.Stream({
| ^^^^^^^^^^^^^^^^^^^
```

Here, I am not sure what is not supported: structs defined in libraries, assignments to mappings, or the `Struct({ key: val })` style of initialing structs?

```text
warning[7650]: Warning: Assertion checker does not yet support this expression.
--> src/SablierV2LockupLinear.sol:419:30:
|
419 | _streams[streamId] = LockupLinear.Stream({
|
```

Ditto.

## Documentation Request

Given the above, it would be helpful to add a new section in the SMTChecker documentation page that would outline the list of currently unsupported features of the compiler.

Start with the linked SMTChecker documentation page and the unsupported-feature warnings shown in the issue. Identify which Solidity features the SMTChecker currently does not support, resolve the uncertainty around structs, mappings, and struct initialization, and add a clear list or section to the documentation page.

Written by the indexing model from the issue text.