docs: add negative-scale @example to stats/base/dists/lognormal/cdf

Adds the `// Negative scale parameter:` `@example` block to the JSDoc
in `lib/main.js` and the `interface CDF` main signature JSDoc in
`docs/types/index.d.ts`. The block documents `cdf( 2.0, 0.0, -1.0 )`
returning `NaN`, which the `sigma <= 0.0` validation guard already
enforces and which the package README and `docs/repl.txt` already
document. The block is present in 4/5 sibling lognormal `(x, mu, sigma)`
packages (`pdf`, `logpdf`, `logcdf`, `quantile`); `cdf` was the lone
outlier at 80% sibling conformance.

Author: claude

lib/node_modules/@stdlib/stats/base/dists/lognormal/cdf/docs/types/index.d.ts

@@ -61,6 +61,11 @@ interface CDF {
 	* @example
 	* var y = cdf( NaN, 0.0, 1.0 );
 	* // returns NaN
+	*
+	* @example
+	* // Negative scale parameter:
+	* var y = cdf( 2.0, 0.0, -1.0 );
+	* // returns NaN
 	*/
 	( x: number, mu: number, sigma: number ): number;
 

lib/node_modules/@stdlib/stats/base/dists/lognormal/cdf/lib/main.js

@@ -54,6 +54,11 @@ var ln = require( '@stdlib/math/base/special/ln' );
 * @example
 * var y = cdf( NaN, 0.0, 1.0 );
 * // returns NaN
+*
+* @example
+* // Negative scale parameter:
+* var y = cdf( 2.0, 0.0, -1.0 );
+* // returns NaN
 */
 function cdf( x, mu, sigma ) {
 	if (