Point Source Replacement to the Overriding Dependencies section

This would've saved me a minute. This mirrors a similarly helpful caveat in [the [patch] section](https://doc.rust-lang.org/cargo/reference/manifest.html#the-patch-section).

diff --git a/src/doc/src/reference/source-replacement.md b/src/doc/src/reference/source-replacement.md
index 20099b29c910309255c3f904a9a6f25b71d1c82b..a70affc7f009d69098e34d1313e2d3107141f7cb 100644 (file)
--- a/src/doc/src/reference/source-replacement.md
+++ b/src/doc/src/reference/source-replacement.md
@@ -1,5 +1,9 @@
 ## Source Replacement
 
+This document is about replacing the crate index. You can read about overriding
+dependencies in the [overriding dependencies][overriding] section of this
+documentation.
+
 Cargo supports the ability to **replace one source with another** to express
 strategies along the lines of mirrors or vendoring dependencies. Configuration
 is currently done through the [`.cargo/config` configuration][config] mechanism,
@@ -56,6 +60,7 @@ dependencies through the usage of [the `[replace]` key][replace-section], and
 private registry support is planned for a future version of Cargo.
 
 [replace-section]: reference/manifest.html#the-replace-section
+[overriding]: reference/specifying-dependencies.html#overriding-dependencies
 
 ### Configuration