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/FindLibssh.cmake

@@ -25,17 +25,17 @@
 #
 # Input variables:
 #
-# LIBSSH_INCLUDE_DIR   The libssh include directory
-# LIBSSH_LIBRARY       Path to libssh library
+# LIBSSH_INCLUDE_DIR   The libssh include directory.
+# LIBSSH_LIBRARY       Path to libssh library.
 #
 # Result variables:
 #
-# LIBSSH_FOUND         System has libssh
-# LIBSSH_INCLUDE_DIRS  The libssh include directories
-# LIBSSH_LIBRARIES     The libssh library names
-# LIBSSH_LIBRARY_DIRS  The libssh library directories
-# LIBSSH_CFLAGS        Required compiler flags
-# LIBSSH_VERSION       Version of libssh
+# LIBSSH_FOUND         System has libssh.
+# LIBSSH_INCLUDE_DIRS  The libssh include directories.
+# LIBSSH_LIBRARIES     The libssh library names.
+# LIBSSH_LIBRARY_DIRS  The libssh library directories.
+# LIBSSH_CFLAGS        Required compiler flags.
+# LIBSSH_VERSION       Version of libssh.
 
 if(CURL_USE_PKGCONFIG AND
    NOT DEFINED LIBSSH_INCLUDE_DIR AND