doc: improve assert strict-mode text

Simplify and clarify introductory material on `strict` mode.
Merge two single-sentence paragraphs into a single paragraph.
Replace "`strict mode`" with "`strict` mode".

PR-URL: #28239
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>

doc/api/assert.md

@@ -5,10 +5,8 @@
 > Stability: 2 - Stable
 
 The `assert` module provides a simple set of assertion tests that can be used to
-test invariants.
-
-A `strict` and a `legacy` mode exist, while it is recommended to only use
-[`strict mode`][].
+test invariants. The module provides a recommended [`strict` mode][] and a more
+lenient `legacy` mode.
 
 For more information about the used equality comparisons see
 [MDN's guide on equality comparisons and sameness][mdn-equality-guide].
@@ -89,7 +87,7 @@ changes:
     description: Added strict mode to the assert module.
 -->
 
-When using the `strict mode`, any `assert` function will use the equality used
+When using the `strict` mode, any `assert` function will use the equality used
 in the strict function mode. So [`assert.deepEqual()`][] will, for example,
 work the same as [`assert.deepStrictEqual()`][].
 
@@ -140,7 +138,7 @@ It can be accessed using:
 const assert = require('assert');
 ```
 
-It is recommended to use the [`strict mode`][] instead as the
+It is recommended to use the [`strict` mode][] instead as the
 [Abstract Equality Comparison][] can often have surprising results. This is
 especially true for [`assert.deepEqual()`][], where the comparison rules are
 lax:
@@ -1288,7 +1286,7 @@ second argument. This might lead to difficult-to-spot errors.
 [`assert.ok()`]: #assert_assert_ok_value_message
 [`assert.strictEqual()`]: #assert_assert_strictequal_actual_expected_message
 [`assert.throws()`]: #assert_assert_throws_fn_error_message
-[`strict mode`]: #assert_strict_mode
+[`strict` mode]: #assert_strict_mode
 [Abstract Equality Comparison]: https://tc39.github.io/ecma262/#sec-abstract-equality-comparison
 [Object wrappers]: https://developer.mozilla.org/en-US/docs/Glossary/Primitive#Primitive_wrapper_objects_in_JavaScript
 [Object.prototype.toString()]: https://tc39.github.io/ecma262/#sec-object.prototype.tostring