build: say Quiche support is experimental, where missing

Follow-up to f2183f51b6 #21795

Closes #21832

CMakeLists.txt

@@ -1167,7 +1167,7 @@ if(USE_NGTCP2)
   list(APPEND CURL_LIBS CURL::nghttp3)
 endif()
 
-option(USE_QUICHE "Use quiche library for HTTP/3 support" OFF)
+option(USE_QUICHE "Use quiche library for HTTP/3 support (experimental)" OFF)
 if(USE_QUICHE)
   if(USE_NGTCP2)
     message(FATAL_ERROR "Only one HTTP/3 backend can be selected")

configure.ac

@@ -3713,8 +3713,8 @@ if test "$disable_http" = "yes" || test "$USE_NGTCP" = "1"; then
 fi
 
 AC_ARG_WITH(quiche,
-AS_HELP_STRING([--with-quiche=PATH],[Enable quiche usage])
-AS_HELP_STRING([--without-quiche],[Disable quiche usage]),
+AS_HELP_STRING([--with-quiche=PATH],[Enable quiche usage (experimental)])
+AS_HELP_STRING([--without-quiche],[Disable quiche usage (experimental)]),
   [OPT_QUICHE=$withval])
 case "$OPT_QUICHE" in
   no)
@@ -3775,7 +3775,7 @@ if test "$want_quiche" != "no"; then
     AC_CHECK_LIB(quiche, quiche_conn_send_ack_eliciting,
       [
         AC_CHECK_HEADERS(quiche.h,
-          experimental="$experimental HTTP3"
+          experimental="$experimental Quiche"
           AC_MSG_NOTICE([HTTP3 support is experimental])
           curl_h3_msg="enabled (quiche)"
           AC_DEFINE(USE_QUICHE, 1, [if quiche is in use])

docs/EXPERIMENTAL.md

@@ -53,6 +53,13 @@ Graduation requirements:
 
 - implementation stability over time with no known severe regressions
 
+### The Quiche backend
+
+Graduation requirements:
+
+- the library needs to consider itself non-beta.
+- a reasonable expectation of a stable API going forward.
+
 ### The Rustls backend
 
 Graduation requirements:

docs/INSTALL-CMAKE.md

@@ -357,7 +357,7 @@ Details via CMake
 - `USE_LIBIDN2`:                            Use libidn2 for IDN support. Default: `ON`
 - `USE_NGHTTP2`:                            Use nghttp2 library. Default: `ON`
 - `USE_NGTCP2`:                             Use ngtcp2 and nghttp3 libraries for HTTP/3 support. Default: `OFF`
-- `USE_QUICHE`:                             Use quiche library for HTTP/3 support. Default: `OFF`
+- `USE_QUICHE`:                             Use quiche library for HTTP/3 support (experimental). Default: `OFF`
 
 ## Dependency options (via CMake)