[9 of 9]: docs: term "logging directives" emphasized (first mention)

Add markdown to emphasize (typical rendering would be italics) the
term "logging directive" upon the first mention in the text.

src/lib.rs

@@ -93,8 +93,8 @@
 //! logging is disabled except for the `error` level**.
 //!
 //! Logging is controlled via the **`RUST_LOG`** environment variable. The
-//! value of this environment variable is a comma-separated list of logging
-//! directives. A logging directive is of the form:
+//! value of this environment variable is a comma-separated list of *logging
+//! directives*. A logging directive is of the form:
 //!
 //! ```text
 //! path::to::module=level