Use the plural for some command line variants

**What is wrong with the docs?**
Lots of project fields have command line variants. These are almost always described in the singular even when they have multiple variants.

The `--keep-going` option has no associated `--no-keep-going` so is truly a singular command line variant.

https://github.com/haskell/cabal/blob/4907eec7b275aa0e1a31333fc745b3cc9f023f4f/doc/cabal-project-description-file.rst?plain=1#L385

Wouldn't it be better grammatically to describe the ones that have multiple command line variants in the plural, like in the diff below?

https://github.com/haskell/cabal/blob/4907eec7b275aa0e1a31333fc745b3cc9f023f4f/doc/cabal-project-description-file.rst?plain=1#L1039-L1040

```diff
$ git diff
diff --git a/doc/cabal-project-description-file.rst b/doc/cabal-project-description-file.rst
index c7e279be3..b9ebee05c 100644
--- a/doc/cabal-project-description-file.rst
+++ b/doc/cabal-project-description-file.rst
@@ -1036,7 +1036,7 @@ feature was added.
even when this value is ``False``; furthermore, test suites are
automatically enabled if they are requested as a built target.

- The command line variant of this flag is ``--enable-tests`` and
+ The command line variants of this flag are ``--enable-tests`` and
``--disable-tests``.
```

Also are the project key value pairs, options, fields or flags? Sections have options, such as [Package configuration options](https://cabal.readthedocs.io/en/latest/cabal-project-description-file.html#package-configuration-options).

Start in doc/cabal-project-description-file.rst, reviewing the command-line variant wording around the linked lines and other project fields with multiple variants. Check how the document names project key-value pairs, options, fields, and flags. Done means the plural wording is consistent where multiple variants exist and the terminology is clarified.

Written by the indexing model from the issue text.