Add URI parsing warning (#21547)

Related to CVE-2021-41259
https://github.com/nim-lang/security/security/advisories/GHSA-3gg2-rw3q-qwgc
https://github.com/nim-lang/Nim/pull/19128#issuecomment-1181944367

lib/pure/httpclient.nim

@@ -10,6 +10,8 @@
 ## This module implements a simple HTTP client that can be used to retrieve
 ## webpages and other data.
 ##
+## .. warning:: Validate untrusted inputs: URI parsers and getters are not detecting malicious URIs.
+##
 ## Retrieving a website
 ## ====================
 ##

lib/pure/uri.nim

@@ -14,6 +14,8 @@
 ## as a locator, a name, or both. The term "Uniform Resource Locator"
 ## (URL) refers to the subset of URIs.
 ##
+## .. warning:: URI parsers in this module do not perform security validation.
+##
 ## # Basic usage