Add CARGO_FUTURE_INCOMPAT_REPORT_FREQUENCY to environment variables

src/doc/src/reference/environment-variables.md

@@ -83,6 +83,7 @@ supported environment variables are:
 * `CARGO_BUILD_DEP_INFO_BASEDIR` — Dep-info relative directory, see [`build.dep-info-basedir`].
 * `CARGO_BUILD_PIPELINING` — Whether or not to use `rustc` pipelining, see [`build.pipelining`].
 * `CARGO_CARGO_NEW_VCS` — The default source control system with [`cargo new`], see [`cargo-new.vcs`].
+* `CARGO_FUTURE_INCOMPAT_REPORT_FREQUENCY` - How often we should generate a future incompat report notifcation, see [`future-incompat-report.frequency`].
 * `CARGO_HTTP_DEBUG` — Enables HTTP debugging, see [`http.debug`].
 * `CARGO_HTTP_PROXY` — Enables HTTP proxy, see [`http.proxy`].
 * `CARGO_HTTP_TIMEOUT` — The HTTP timeout, see [`http.timeout`].
@@ -144,6 +145,7 @@ supported environment variables are:
 [`cargo-new.name`]: config.md#cargo-newname
 [`cargo-new.email`]: config.md#cargo-newemail
 [`cargo-new.vcs`]: config.md#cargo-newvcs
+[`future-incompat-report.frequency`]: config.md#future-incompat-reportfrequency
 [`http.debug`]: config.md#httpdebug
 [`http.proxy`]: config.md#httpproxy
 [`http.timeout`]: config.md#httptimeout