Rollup merge of rust-lang#151227 - Kobzol:document-cache-proc-macros, r=jackh726

Document `-Zcache-proc-macros`

Forgot about this in rust-lang#145354.

Created a tracking issue in rust-lang#151364.

Author: Zalathar

src/doc/unstable-book/src/compiler-flags/cache-proc-macros.md

@@ -0,0 +1,9 @@
+## `cache-proc-macros`
+
+The tracking issue for this feature is: [#151364]
+
+[#151364]: https://github.com/rust-lang/rust/issues/151364
+
+------------------------
+
+This option instructs `rustc` to cache (derive) proc-macro invocations using the incremental system. Note that the compiler does not currently check whether the proc-macro is actually "cacheable" or not. If you use this flag when compiling a crate that uses non-pure proc-macros, it can result in stale expansions being compiled.