libcurl-env-dbg.3: move debug variables from libcurl-env.3

- Move documentation of libcurl environment variables used only in debug
  builds from libcurl-env into a separate document libcurl-env-dbg.

- Document more debug environment variables.

Previously undocumented or missing a description:

CURL_ALTSVC_HTTP, CURL_DBG_SOCK_WBLOCK, CURL_DBG_SOCK_WPARTIAL,
CURL_DBG_QUIC_WBLOCK, CURL_DEBUG, CURL_DEBUG_SIZE, CURL_GETHOSTNAME,
CURL_HSTS_HTTP, CURL_FORCETIME, CURL_SMALLREQSEND, CURL_SMALLSENDS,
CURL_TIME.

Closes https://github.com/curl/curl/pull/11811

.github/scripts/spellcheck.words

@@ -14,6 +14,7 @@ allocator
 alnum
 ALPN
 Altera
+AltSvc
 ALTSVC
 amiga
 AmigaOS
@@ -139,6 +140,7 @@ cURL
 CURLcode
 CURLE
 CURLH
+CURLINFO
 curlimages
 curlrc
 curltest
@@ -151,7 +153,9 @@ cyassl
 Cygwin
 daniel
 datatracker
+dbg
 Debian
+DEBUGBUILD
 decrypt
 deepcode
 DELE