cmake: document -D and env build options

Extend `INSTALL-CMAKE` document with the list of available options,
a short description and default values.

The list may not be 100% complete.

There are no component boundaries in CMake, so the line is blurry
between curl options, CMake options, CMake Find modules options.
I included certain CMake options that seemed useful, and/or have
dedicated use withing curl's CMake source. But, all CMake built-in
options are usable, as documented upstream in CMake.

The naming of the options has a heritage and the inconsistencies with
it, including a lack of clear namespace. This may be subject to future
updates, also after figuring out which name has special meaning within
CMake and/or CMake projects out of unwritten convention or something
more tangible.

CMake allows to initialize any internal variable via `-D`. This may be
useful to pre-initialize/override feature check results. The list
doesn't contain these, and they remain officially undocumented.

Also:
- make adjustments to keep the spellchecker happy.
- retrofit description changes to the cmake sources.
- stop documenting deprecated `Find*` variables.

Reported-by: Daniel Stenberg
Fixes https://github.com/curl/curl/discussions/14885
Closes #15388

CMake/FindLibpsl.cmake

@@ -25,15 +25,15 @@
 #
 # Input variables:
 #
-# LIBPSL_INCLUDE_DIR   The libpsl include directory
-# LIBPSL_LIBRARY       Path to libpsl library
+# - `LIBPSL_INCLUDE_DIR`:   The libpsl include directory.
+# - `LIBPSL_LIBRARY`:       Path to `libpsl` library.
 #
 # Result variables:
 #
-# LIBPSL_FOUND         System has libpsl
-# LIBPSL_INCLUDE_DIRS  The libpsl include directories
-# LIBPSL_LIBRARIES     The libpsl library names
-# LIBPSL_VERSION       Version of libpsl
+# - `LIBPSL_FOUND`:         System has libpsl.
+# - `LIBPSL_INCLUDE_DIRS`:  The libpsl include directories.
+# - `LIBPSL_LIBRARIES`:     The libpsl library names.
+# - `LIBPSL_VERSION`:       Version of libpsl.
 
 if(CURL_USE_PKGCONFIG)
   find_package(PkgConfig QUIET)