internals/backend: refreshed, added HTTP/3 backend info

Closes #492

internals/backends.md

@@ -22,16 +22,16 @@ functionality. In these different areas there are multiple different providers:
 2. Name resolving
 3. TLS
 4. SSH
-5. HTTP/1 and HTTP/2
+5. HTTP/1
 6. HTTP/3
 7. HTTP content encoding
 
 ## Backends visualized
 
 ![libcurl backends](slide-libcurl-backends.jpg)
 
-Applications (in the upper yellow cloud) access libcurl through the public
-API. The API is fixed and stable.
+Applications (in the upper green cloud) access libcurl through the public API.
+The API is fixed and stable.
 
 Internally, the core of libcurl uses internal APIs to perform the different
 duties it needs to do. Each of these internal APIs are powered by alternative
@@ -40,3 +40,21 @@ implementations, in many times powered by different third party libraries.
 The image above shows the different third party libraries powering different
 internal APIs. The purple boxes are one or more and the dark gray ones are
 "one of these".
+
+## HTTP/3 backends
+
+![HTTP/3 backends](slide-http3-backends.jpg)
+
+libcurl offers four different backends, visualized as different column in this
+picture.
+
+The same data in a table, where each line is a specific backend build:
+
+| status       | HTTP/3 library | QUIC library   | TLS libraries (one of)         |
+|--------------|----------------|----------------|--------------------------------|
+| recommended  | nghttp3        | ngtcp2         | fork family, GnuTLS or wolfSSL |
+| experimental | quiche         | quiche         | BoringSSL or quictls           |
+| experimental | msh3           | msquic         | fork family or Schannel        |
+| experimental | nghttp3        | OpenSSL 3.3.0+ | OpenSSL 3.3.0+                 |
+
+*fork family* means BoringSSL, LibreSSL, quictls and AWS-LC

wordlist.txt

@@ -589,3 +589,5 @@ Zakrzewski
 Zitzmann
 zlib
 zsh
+msquic
+quictls