docs: update example variables and document domain

PR-URL: #12301
Reviewed-by: Athan Reines <kgryte@gmail.com>

Author: Planeshifter

lib/node_modules/@stdlib/stats/base/dists/chisquare/mean/README.md

@@ -41,7 +41,7 @@ The [expected value][expected-value] for a [chi-squared][chisquare-distribution]
 
 <!-- </equation> -->
 
-where `k` is the degrees of freedom.
+where `k >= 0` is the degrees of freedom.
 
 </section>
 

lib/node_modules/@stdlib/stats/base/dists/chisquare/median/README.md

@@ -55,7 +55,7 @@ v = median( 2.0 );
 If provided `k < 0`, the function returns `NaN`.
 
 ```javascript
-var y = median( -1.0 );
+var v = median( -1.0 );
 // returns NaN
 ```
 

lib/node_modules/@stdlib/stats/base/dists/chisquare/median/docs/types/index.d.ts

@@ -37,11 +37,11 @@
 * // returns ~1.386
 *
 * @example
-* var k = median( -0.2 );
+* var v = median( -0.2 );
 * // returns NaN
 *
 * @example
-* var k = median( NaN );
+* var v = median( NaN );
 * // returns NaN
 */
 declare function median( k: number ): number;