docs: Clarify git sources vs git registries in source replacement documentation (#15974)

### What does this PR try to resolve?

This PR addresses confusion in the documentation around git sources and
git registries that has been causing frustrating user experiences. The
current documentation is unclear about the purpose of git sources and
doesn't mention that they're not usable for replacing registries,
leading users to mistakenly assume git sources can replace registries
using the git protocol.

Fixes #15971

### How to test and review this PR?

- Documentation build verification: Ensure the markdown renders
correctly and all cross-references work
- Link validation: Verify all new reference links point to existing
documentation sections
- Content accuracy: Confirm technical explanations are correct and align
with Cargo's actual behavior

Author: epage

src/doc/src/reference/source-replacement.md

@@ -1,8 +1,12 @@
 # Source Replacement
 
-This document is about replacing the crate index. You can read about overriding
-dependencies in the [overriding dependencies] section of this
-documentation.
+This document is about redirecting communication with [registries]
+or repositories of [git-based dependencies] to another data source, such as a
+server mirroring the original registry or an exact local copy.
+
+If you want to patch individual dependencies, see [overriding dependencies] section of this
+documentation. If you want to control how Cargo makes network requests, see [`[http]`](config.md#http)
+and [`[net]`](config.md#net) configuration.
 
 A *source* is a provider that contains crates that may be included as
 dependencies for a package. Cargo supports the ability to **replace one source
@@ -87,13 +91,26 @@ git = "https://example.com/path/to/repo"
 
 ## Registry Sources
 
-A "registry source" is one that is the same as crates.io itself. That is, it has
-an index served in a git repository which matches the format of the
-[crates.io index](https://github.com/rust-lang/crates.io-index). That repository
-then has configuration indicating where to download crates from.
+A "registry source" is one that works like crates.io itself. It's an index
+that conforms to the specification at https://doc.rust-lang.org/cargo/reference/registry-index.html
+with a configuration file indicating where to download crates from.
+
+Registry sources can use [either git or sparse HTTP protocol][protocols]:
+
+```toml
+# Git protocol
+registry = "ssh://git@example.com/path/to/index.git"
 
-Currently there is not an already-available project for setting up a mirror of
-crates.io. Stay tuned though!
+# Sparse HTTP protocol  
+registry = "sparse+https://example.com/path/to/index"
+
+# HTTPS git protocol
+registry = "https://example.com/path/to/index"
+```
+
+[protocols]: registries.md#registry-protocols
+
+[crates.io index]: https://doc.rust-lang.org/cargo/reference/registry-index.html
 
 ## Local Registry Sources
 
@@ -131,3 +148,13 @@ is placed on the name of each directory.
 Each crate in a directory source also has an associated metadata file indicating
 the checksum of each file in the crate to protect against accidental
 modifications.
+
+## Git sources
+
+Git sources represent repositories used by [git-based dependencies]. They're
+used to specify which git-based dependencies should be replaced with alternative sources.
+
+Git sources are *not* related to the [git registries][protocols],
+and can't be used to replace registry sources.
+
+[git-based dependencies]: specifying-dependencies.md#specifying-dependencies-from-git-repositories