`os()` platform condition incorrectly defined in prose and language grammar

[grynspan]

Location

swift-book/TSPL.docc/ReferenceManual/Statements.md

Line 1007 in d293687

| `os()` | `macOS`, `iOS`, `watchOS`, `tvOS`, `visionOS`, `Linux`, `Windows` |

swift-book/TSPL.docc/ReferenceManual/SummaryOfTheGrammar.md

Line 701 in d293687

> *operating-system* → **`macOS`** | **`iOS`** | **`watchOS`** | **`tvOS`** | **`visionOS`** | **`Linux`** | **`Windows`** \

Description

The lists of valid OS names used with os() are incomplete, as the language supports a variety of other OS names such as WASI and FreeBSD.

Correction

We should either leave the list open-ended, or try to comprehensively list everything the compiler supports.

It's also worth noting that if a developer passes some unsupported OS name, it's not a compiler error—the compiler will accept it, emit a warning, and just ignore the code as if it said false instead of os(whatever). So perhaps the grammar should reflect that, if possible.

[amartini51]

TSPL documents only the platforms that have official support. If some are missing, yes we should add them.

For additional discussion, please see #225 and #245.

[grynspan]

The grammar of the language allows for additional values even if they're not actually supported.

[grynspan]

It would be sufficient to adjust the formal grammar to allow for any identifier token, with a comment to the effect of "these are the supported values; other values will compile but produce diagnostics".