docs: update pypi part (#546)

Closes #500

docs/cli.md

@@ -36,8 +36,11 @@ It will only add if the package with its version constraint is able to work with
 ##### Options
 
 - `--manifest-path`: the path to `pixi.toml`, by default it searches for one in the parent directories.
-- `--host`: Specify that it is a host dependency, important for building a package.
-- `--build`: Specify that it is a build dependency, important for building a package.
+- `--host`: Specifies a host dependency, important for building a package.
+- `--build`: Specifies a build dependency, important for building a package.
+- `--pypi`: Specifies a PyPI dependency, not a conda package.
+      Parses dependencies as [PEP508](https://peps.python.org/pep-0508/) requirements, supporting extras and versions.
+      See [configuration](configuration.md) for details.
 - `--no-install`: Don't install the package to the environment, only add the package to the lock-file.
 - `--platform (-p)`: The platform for which the dependency should be added. (Allowed to be used more than once)
 
@@ -48,6 +51,7 @@ pixi add "numpy>=1.22,<1.24"
 pixi add --manifest-path ~/myproject/pixi.toml numpy
 pixi add --host "python>=3.9.0"
 pixi add --build cmake
+pixi add --pypi requests[security]
 pixi add --platform osx-64 --build clang
 ```
 

docs/configuration.md

@@ -246,23 +246,24 @@ These are not available on [prefix.dev](https://prefix.dev/channels) but on [pyp
 !!! warning "Important considerations"
     - **Stability**: PyPI packages might be less stable than their conda counterparts. Prefer using conda packages in the `dependencies` table where possible.
     - **Compatibility limitations**: Currently, pixi doesn't support:
-        - `git` dependencies
+        - `git` dependencies (`git+https://github.com/package-org/package.git`)
         - Source dependencies
         - Private PyPI repositories
     - **Version specification**: These dependencies don't follow the conda matchspec specification.
+    The `version` is a [`VersionSpecifier`](https://docs.rs/pep440_rs/0.3.12/pep440_rs/struct.VersionSpecifiers.html) and the `extras` are a list of `Strings`.
     So see the example below to see what type of definition is allowed.
-    It is based on [pep440](https://peps.python.org/pep-0440/)
+
 
 ```toml
 [dependencies]
 python = ">=3.6" # Python is needed for the pypi dependencies!
 
 [pypi-dependencies]
-pytest = "==7.4.3"
-torch = "*"  # This means any version (this `*` is custom in pixi)
-pre-commit = "~=3.5.0"
-numpy = ">=1.20,<1.24"
+pytest = "*"  # This means any version (this `*` is custom in pixi)
+pre-commit = "~=3.5.0" # Single string is of type VersionSpecifiers
+requests = {version = ">= 2.8.1, ==2.8.*", extras=["security", "tests"]} # Using the map allows the user to add `extras`
 ```
+
 ??? info "We use `rip` not `pip`"
     We use [`rip`](https://github.com/prefix-dev/rip) which is our custom pypi package resolver.
     The `rip` resolve step is invoked after the conda dependencies have been resolved.