docs: fix comment

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---

Author: headlessNode

lib/node_modules/@stdlib/blas/ext/base/ndarray/gfind-index/README.md

@@ -55,7 +55,7 @@ var idx = gfindIndex( [ x ], isEven );
 // returns 2
 ```
 
-If the function is unable to find a search element, the function returns `-1`.
+If no element passes a test implemented by a predicate function, the function returns `-1`.
 
 ```javascript
 var scalar2ndarray = require( '@stdlib/ndarray/from-scalar' );
@@ -116,7 +116,6 @@ var count = ctx.count;
 ## Notes
 
 -   If provided an empty one-dimensional ndarray, the function returns `-1`.
--   A provided callback function should return a boolean value.
 
 </section>
 

lib/node_modules/@stdlib/blas/ext/base/ndarray/gfind-index/docs/repl.txt

@@ -3,16 +3,15 @@
     Returns the index of the first element in a one-dimensional ndarray which
     passes a test implemented by a predicate function.
 
-    If provided an empty ndarray, the function returns `-1`.
+    If provided an empty ndarray or no element passes a test implemented by a
+    predicate function, the function returns `-1`.
 
     The callback function is provided three arguments:
 
     - value: current array element.
     - index: current array index.
     - array: the input ndarray.
 
-    The callback function should return a boolean value.
-
     Parameters
     ----------
     arrays: ArrayLikeObject<ndarray>