page-footer: document more environment variables

... that curl might use.

Closes #8027

docs/cmdline-opts/page-footer

@@ -40,6 +40,18 @@ versions should then be given without enclosing brackets.
 IPv6 numerical addresses are compared as strings, so they will only match if
 the representations are the same: "::1" is the same as "::0:1" but they do not
 match.
+.IP "APPDATA <dir>"
+On Windows, this variable is used when trying to find the home directory. If
+the primary home variable are all unset.
+.IP "COLUMNS <terminal width>"
+If set, the specified number of characters will be used as the terminal width
+when the alternative progress-bar is shown. If not set, curl will try to
+figure it out using other ways.
+.IP "CURL_CA_BUNDLE <file>"
+If set, will be used as the \fI--cacert\fP value.
+.IP "CURL_HOME <dir>"
+If set, is the first variable curl checks when trying to find its home
+directory. If not set, it continues to check \fBXDG_CONFIG_HOME\fP.
 .IP "CURL_SSL_BACKEND <TLS backend>"
 If curl was built with support for "MultiSSL", meaning that it has built-in
 support for more than one TLS backend, this environment variable can be set to
@@ -49,17 +61,34 @@ stay with the default.
 
 SSL backend names (case-insensitive): bearssl, gnutls, gskit, mbedtls,
 mesalink, nss, openssl, rustls, schannel, secure-transport, wolfssl
+.IP "HOME <dir>"
+If set, this is used to find the home directory when that is needed. Like when
+looking for the default .curlrc. \fBCURL_HOME\fP and \fBXDG_CONFIG_HOME\fP
+have preference.
 .IP "QLOGDIR <directory name>"
 If curl was built with HTTP/3 support, setting this environment variable to a
 local directory will make curl produce qlogs in that directory, using file
 names named after the destination connection id (in hex). Do note that these
 files can become rather large. Works with both QUIC backends.
+.IP SHELL
+Used on VMS when trying to detect if using a DCL or a "unix" shell.
+.IP "SSL_CERT_DIR <dir">
+If set, will be used as the \fI--capath\fP value.
+.IP "SSL_CERT_FILE <path>"
+If set, will be used as the \fI--cacert\fP value.
 .IP "SSLKEYLOGFILE <file name>"
 If you set this environment variable to a file name, curl will store TLS
 secrets from its connections in that file when invoked to enable you to
 analyze the TLS traffic in real time using network analyzing tools such as
 Wireshark. This works with the following TLS backends: OpenSSL, libressl,
 BoringSSL, GnuTLS, NSS and wolfSSL.
+.IP "USERPROFILE <dir>"
+On Windows, this variable is used when trying to find the home directory. If
+the other, primary, variable are all unset. If set, curl will use the path
+"$USERPROFILE\\Application Data".
+.IP "XDG_CONFIG_HOME <dir>"
+If \fBCURL_HOME\fP is not set, this variable is checked when looking for a
+default .curlrc file.
 .SH "PROXY PROTOCOL PREFIXES"
 The proxy string may be specified with a protocol:// prefix to specify
 alternative proxy protocols. (Added in 7.21.7)