doc: add pending-deprecation to deprecations list

This adds a mention of `--pending-deprecation` flag effect
in the Deprecated APIs document, and explicitly labels
deprecations that support it.

Refs: #18417
PR-URL: #18433
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>

doc/api/deprecations.md

@@ -14,6 +14,12 @@ Node.js utilizes three kinds of Deprecations:
 
 A Documentation-only deprecation is one that is expressed only within the
 Node.js API docs. These generate no side-effects while running Node.js.
+Some Documentation-only deprecations trigger a runtime warning when launched
+with [`--pending-deprecation`][] flag (or its alternative,
+`NODE_PENDING_DEPRECATION=1` environment variable), similarly to Runtime
+deprecations below. Documentation-only deprecations that support that flag
+are explicitly labeled as such in the
+[list of Deprecated APIs](#deprecations_list_of_deprecated_apis).
 
 A Runtime deprecation will, by default, generate a process warning that will
 be printed to `stderr` the first time the deprecated API is used. When the
@@ -66,7 +72,7 @@ be used.
 <a id="DEP0005"></a>
 ### DEP0005: Buffer() constructor
 
-Type: Documentation-only
+Type: Documentation-only (supports [`--pending-deprecation`][])
 
 The `Buffer()` function and `new Buffer()` constructor are deprecated due to
 API usability issues that can potentially lead to accidental security issues.
@@ -780,6 +786,7 @@ Use [`asyncResource.runInAsyncScope()`][] API instead which provides a much
 safer, and more convenient, alternative. See
 https://github.com/nodejs/node/pull/18513 for more details.
 
+[`--pending-deprecation`]: cli.html#cli_pending_deprecation
 [`Buffer.allocUnsafeSlow(size)`]: buffer.html#buffer_class_method_buffer_allocunsafeslow_size
 [`Buffer.from(array)`]: buffer.html#buffer_class_method_buffer_from_array
 [`Buffer.from(buffer)`]: buffer.html#buffer_class_method_buffer_from_buffer