Document -Zcache-proc-macros

Author: Kobzol

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

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+## `cache-proc-macros`
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+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.