doc(man): add temp-target-dir variable

Author: weihanglo

src/doc/man/cargo-install.md

@@ -1,5 +1,6 @@
 # cargo-install(1)
 {{*set actionverb="Install"}}
+{{*set temp-target-dir=true}}
 
 ## NAME
 

src/doc/man/includes/options-target-dir.md

@@ -1,6 +1,9 @@
 {{#option "`--target-dir` _directory_"}}
 Directory for all generated artifacts and intermediate files. May also be
 specified with the `CARGO_TARGET_DIR` environment variable, or the
-`build.target-dir` [config value](../reference/config.html). Defaults
-to `target` in the root of the workspace.
+`build.target-dir` [config value](../reference/config.html).
+{{#if temp-target-dir}} Defaults to a new temporary folder located in the
+temporary directory of the platform.
+{{else}} Defaults to `target` in the root of the workspace.
+{{/if}}
 {{/option}}