[NFC][core] add ROOTENV_USER_PATH to TEnv class doc

(cherry picked from commit 8aa4000)

Author: jblomer

core/base/src/TEnv.cxx

@@ -16,11 +16,15 @@ The TEnv class reads config files, by default named `.rootrc`.
 Three types of config files are read: global, user and local files. The
 global file is `$ROOTSYS/etc/system<name>` (or `ROOTETCDIR/system<name>`)
 the user file is `$HOME/<name>` and the local file is `./<name>`.
+
 By setting the shell variable `ROOTENV_NO_HOME=1` the reading of
 the `$HOME/<name>` resource file will be skipped. This might be useful
 in case the home directory resides on an auto-mounted remote file
 system and one wants to avoid this file system from being mounted.
 
+By setting ROOTENV_USER_PATH=<path>, the user-level configuration can
+be read from a custom path instead of from $HOME.
+
 The format of the `.rootrc` file is similar to the `.Xdefaults` format:
 ~~~ {.cpp}
   [+]<SystemName>.<RootName|ProgName>.<name>[(type)]:  <value>
@@ -388,18 +392,18 @@ TString TEnvRec::ExpandValue(const char *value)
 
 ////////////////////////////////////////////////////////////////////////////////
 /// Create a resource table and read the (possibly) three resource files,
-/// i.e.\ `$ROOTSYS/etc/system<name>` or `ROOTETCDIR/system<name>` 
+/// i.e.\ `$ROOTSYS/etc/system<name>` or `ROOTETCDIR/system<name>`
 /// (kEnvGlobal), `$HOME/<name>` or (kEnvUser),  and `$PWD/<name>` (kEnvLocal).
 /// ROOT always reads ".rootrc" (in TROOT::InitSystem()). You can
 /// read additional user defined resource files by creating additional TEnv
 /// objects. By setting the shell variable ROOTENV_NO_HOME=1 the reading of
 /// the `$HOME/<name>` resource file will be skipped. This might be useful in
 /// case the home directory resides on an auto-mounted remote file system
 /// and one wants to avoid the file system from being mounted.
-/// In case the environment variable ROOTENV_USER_PATH is specified, 
-/// and ROOTENV_NO_HOME is not set, then `$ROOTENV_USER_PATH/<name>` 
+/// In case the environment variable ROOTENV_USER_PATH is specified,
+/// and ROOTENV_NO_HOME is not set, then `$ROOTENV_USER_PATH/<name>`
 /// is considered instead of `$HOME/<name>`.
-/// If environment variables have to be avoided, a `rootlogon.C` script 
+/// If environment variables have to be avoided, a `rootlogon.C` script
 /// can be created where where the environment can be set through an
 /// invocation of TEnv::ReadFile.