docs: lots of minor language polish

Mostly based on recent language decisions from "everything curl":

- remove contractions (isn't => is not)
- *an* HTTP (consistency)
- runtime (no hyphen)
- backend (no hyphen)
- URL is uppercase

Closes #8646

docs/libcurl/libcurl-tutorial.3

@@ -5,7 +5,7 @@
 .\" *                            | (__| |_| |  _ <| |___
 .\" *                             \___|\___/|_| \_\_____|
 .\" *
-.\" * Copyright (C) 1998 - 2021, Daniel Stenberg, <daniel@haxx.se>, et al.
+.\" * Copyright (C) 1998 - 2022, Daniel Stenberg, <daniel@haxx.se>, et al.
 .\" *
 .\" * This software is licensed as described in the file COPYING, which
 .\" * you should have received as part of this distribution. The terms
@@ -131,8 +131,8 @@ Repeated calls to \fIcurl_global_init(3)\fP and \fIcurl_global_cleanup(3)\fP
 should be avoided. They should only be called once each.
 
 .SH "Features libcurl Provides"
-It is considered best-practice to determine libcurl features at run-time
-rather than at build-time (if possible of course). By calling
+It is considered best-practice to determine libcurl features at runtime rather
+than at build-time (if possible of course). By calling
 \fIcurl_version_info(3)\fP and checking out the details of the returned
 struct, your program can figure out exactly what the currently running libcurl
 supports.