docs: Improve AUGUR_DEBUG description

Includes clarification of "a truthy value" as "a non-empty value", since
all env vars values are strings and any non-empty string is a truthy
value in Python.  The new phrasing matches existing phrasing in our docs
and makes it much clearer to a reader not thinking about details of
Python's values.

Author: tsibley

CHANGES.md

@@ -6,7 +6,7 @@
 
 * A new command, `augur merge`, now allows for generalized merging of two or more metadata tables. [#1563][] (@tsibley)
 * Two new commands, `augur read-file` and `augur write-file`, now allow external programs to do i/o like Augur by piping from/to these new commands.  They provide handling of compression formats and newlines consistent with the rest of Augur. [#1562][] (@tsibley)
-* A new debugging mode can be enabled by setting the `AUGUR_DEBUG` environment variable to `1` (or another truthy value).  Currently the only effect is to print more information about handled (i.e. anticipated) errors.  For example, stack traces and parent exceptions in an exception chain are normally omitted for handled errors, but setting this env var includes them.  Future debugging and troubleshooting features, like verbose operation logging, will likely also condition on this new debugging mode. [#1577][] (@tsibley)
+* A new debugging mode can be enabled by setting the `AUGUR_DEBUG` environment variable to `1` (or any non-empty value).  Currently the only effect is to print more information about handled (i.e. anticipated) errors.  For example, stack traces and parent exceptions in an exception chain are normally omitted for handled errors, but setting this env var includes them.  Future debugging and troubleshooting features, like verbose operation logging, will likely also condition on this new debugging mode. [#1577][] (@tsibley)
 
 ### Bug Fixes
 

docs/usage/envvars.rst

@@ -8,9 +8,10 @@ These can be especially useful in the context of an entire pipeline or workflow
 .. envvar:: AUGUR_DEBUG
 
     Boolean.
-    Set to a truthy value (e.g. 1) to print more information about (handled) errors.
+    If set to a non-empty value, more detailed debugging information is shown by Augur during execution and handling of errors.
 
-    For example, when this is not set or falsey, stack traces and parent exceptions in an exception chain are omitted from handled errors.
+    For example, when this is enabled, stack traces and parent exceptions in an exception chain are no longer omitted from handled (i.e. expected) errors.
+    Some commands will also emit more verbose operation logging during their execution.
 
 .. envvar:: AUGUR_MINIFY_JSON