docs(metadata): Added build_directory to cargo metadata documentation (#15410)

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### What does this PR try to resolve?

Follow up to #15377 that documents `build_directory` in the cargo
metadata docs.

I did not mention that this feature is not stable yet, let me know if it
would be better to a note that this is nightly only.
Not sure how we normally handle documenting non-stable features

cc: #14125

Author: weihanglo

src/doc/man/cargo-metadata.md

@@ -318,8 +318,10 @@ The JSON output has the following format:
         */
         "root": "file:///path/to/my-package#0.1.0",
     },
-    /* The absolute path to the build directory where Cargo places its output. */
+    /* The absolute path to the target directory where Cargo places its output. */
     "target_directory": "/path/to/my-package/target",
+    /* The absolute path to the build directory where Cargo places intermediate build artifacts. (unstable) */
+    "build_directory": "/path/to/my-package/build-dir",
     /* The version of the schema for this metadata structure.
        This will be changed if incompatible changes are ever made.
     */

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

@@ -314,8 +314,10 @@ OUTPUT FORMAT
                    */
                    "root": "file:///path/to/my-package#0.1.0",
                },
-               /* The absolute path to the build directory where Cargo places its output. */
+               /* The absolute path to the target directory where Cargo places its output. */
                "target_directory": "/path/to/my-package/target",
+               /* The absolute path to the build directory where Cargo places intermediate build artifacts. (unstable) */
+               "build_directory": "/path/to/my-package/build-dir",
                /* The version of the schema for this metadata structure.
                   This will be changed if incompatible changes are ever made.
                */

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

@@ -318,8 +318,10 @@ The JSON output has the following format:
         */
         "root": "file:///path/to/my-package#0.1.0",
     },
-    /* The absolute path to the build directory where Cargo places its output. */
+    /* The absolute path to the target directory where Cargo places its output. */
     "target_directory": "/path/to/my-package/target",
+    /* The absolute path to the build directory where Cargo places intermediate build artifacts. (unstable) */
+    "build_directory": "/path/to/my-package/build-dir",
     /* The version of the schema for this metadata structure.
        This will be changed if incompatible changes are ever made.
     */

src/etc/man/cargo-metadata.1

@@ -320,8 +320,10 @@ The JSON output has the following format:
         */
         "root": "file:///path/to/my\-package#0.1.0",
     },
-    /* The absolute path to the build directory where Cargo places its output. */
+    /* The absolute path to the target directory where Cargo places its output. */
     "target_directory": "/path/to/my\-package/target",
+    /* The absolute path to the build directory where Cargo places intermediate build artifacts. (unstable) */
+    "build_directory": "/path/to/my\-package/build\-dir",
     /* The version of the schema for this metadata structure.
        This will be changed if incompatible changes are ever made.
     */