docs: Clarify vendored sources as read-only and way to modify

src/doc/man/cargo-vendor.md

@@ -21,6 +21,11 @@ stdout after `cargo vendor` completes the vendoring process.
 You will need to add or redirect it to your Cargo configuration file,
 which is usually `.cargo/config.toml` locally for the current package.
 
+By default, Cargo treats vendored crates as read-only as it does crates.io.
+To modify a vendored crate the correct way is to use `[patch]` or a `path` dependency,
+and Cargo will then correctly handle the crate on incremental rebuilds as it knowns
+that it is not a read-only dependency.
+
 ## OPTIONS
 
 ### Vendor Options

src/doc/man/generated_txt/cargo-vendor.txt

@@ -18,6 +18,11 @@ DESCRIPTION
        need to add or redirect it to your Cargo configuration file, which is
        usually .cargo/config.toml locally for the current package.
 
+       By default, Cargo treats vendored crates as read-only as it does
+       crates.io. To modify a vendored crate the correct way is to use [patch]
+       or a path dependency, and Cargo will then correctly handle the crate on
+       incremental rebuilds as it knowns that it is not a read-only dependency.
+
 OPTIONS
    Vendor Options
        -s manifest, --sync manifest

src/doc/src/commands/cargo-vendor.md

@@ -21,6 +21,11 @@ stdout after `cargo vendor` completes the vendoring process.
 You will need to add or redirect it to your Cargo configuration file,
 which is usually `.cargo/config.toml` locally for the current package.
 
+By default, Cargo treats vendored crates as read-only as it does crates.io.
+To modify a vendored crate the correct way is to use `[patch]` or a `path` dependency,
+and Cargo will then correctly handle the crate on incremental rebuilds as it knowns
+that it is not a read-only dependency.
+
 ## OPTIONS
 
 ### Vendor Options

src/etc/man/cargo-vendor.1

@@ -18,6 +18,11 @@ The configuration necessary to use the vendored sources would be printed to
 stdout after \fBcargo vendor\fR completes the vendoring process.
 You will need to add or redirect it to your Cargo configuration file,
 which is usually \fB\&.cargo/config.toml\fR locally for the current package.
+.sp
+By default, Cargo treats vendored crates as read\-only as it does crates.io.
+To modify a vendored crate the correct way is to use \fB[patch]\fR or a \fBpath\fR dependency,
+and Cargo will then correctly handle the crate on incremental rebuilds as it knowns
+that it is not a read\-only dependency.
 .SH "OPTIONS"
 .SS "Vendor Options"
 .sp