Improve the description of the variables

Author: seisman

doc/techref/environment_variables.md

@@ -1,38 +1,47 @@
 # Environmental Variables
 
-Several environment variables can be used to control the behavior of PyGMT. These
-environment variables can be set in your shell or in your Python script using the
+PyGMT's behavior can be controlled through various environment variables. These variables
+can be set either in your shell environment or within your Python script using the
 :py:data:`os.environ` dictionary.
 
-Here we list the environment variables that are used by PyGMT. The environment
-variables are divided into three categories: system environment variables, GMT/PyGMT
-environment variables, and module-specific environment variables.
+Here we list the environment variables used by PyGMT. The environment variables are
+categorized into three groups:
+
+1. System environment variables
+2. GMT/PyGMT environment variables
+3. Module-specific environment variables
 
 ## System Environment Variables
 
 ```{glossary}
 TZ
-    Specify the time zone for the current calendar time. Refer to the
-    [Specifying the Time Zone with TZ](https://www.gnu.org/software/libc/manual/html_node/TZ-Variable.html)
-    for the valid format.
+    Specify the time zone for the current calendar time. It can be set to a string that
+    defines the timezone, such as `"UTC"`, `"America/New_York"`, or `"Europe/London"`.
+    Refer to the [Specifying the Time Zone with TZ](https://www.gnu.org/software/libc/manual/html_node/TZ-Variable.html)
+    for the valid format. If not set, the system's default timezone is used.
 ```
 
 ## GMT/PyGMT Environment Variables
 
 ```{glossary}
 GMT_LIBRARY_PATH
-    Directory to the GMT library.
+    Specify the directory where the GMT shared library is located. This is useful when
+    GMT is installed in a non-standard location or when you want to use a specific
+    version of GMT. If not set, PyGMT will attempt to find the GMT library in standard
+    system locations.
 
 PYGMT_USE_EXTERNAL_DISPLAY
-    Setting this environment variable to `"false"` can disable image preview in
-    external viewers. It's useful when running the tests and building the documentation
-    to avoid popping up windows.
+    Whether to use external viewers for displaying images. If set to `"false"`, PyGMT
+    will not attempt to open images in external viewers. This can be useful when running
+    tests or building the documentation to avoid popping up windows.
 ```
 
 ## Module-Specific Environment Variables
 
 ```{glossary}
 X2SYS_HOME
-    Specify the directory where the x2sys-related functions (e.g.,
-    {func}`pygmt.x2sys_init`) can keep track of the x2sys settings.
+    Specify the directory where x2sys databases and related settings will be stored.
+    This environment variable is used by x2sys-related functions (e.g.,
+    {func}`pygmt.x2sys_init`) to manage and access x2sys data. If not set, these
+    functions will use a default directory or prompt for a location.
 ```