doc: --config-override, PANTS_ environment vars

Wrote a blurb --config-override, since I'd guessed wrong about the behavior of PANTS_CONFIG_OVERRIDE back in the day.

Wrote about PANTS_... environment variables.
Made some guesses about which parts to write about.
Linked to options.py for folks who want the whole story.

Bugs closed: 661, 847

Reviewed at https://rbcommons.com/s/twitter/r/1640/

src/docs/invoking.md

@@ -59,3 +59,79 @@ passthrough args to tools called by all of `test`, not just `test.pytest`.
 This gets tricky. We happen to know we can pass `-k list` here, that `pytest` accepts passthrough
 args, and that this `test` invocation upon `tests/python/...` won't invoke any other tools
 (assuming nobody hid a `junit` test in the `python` directory).
+
+`PANTS_...` Environment Variables
+---------------------------------
+
+Each goal option also has a corresponding environment variable. For example, either of these
+commands opens a coverage report in your browser:
+
+    :::bash
+    $ ./pants test.junit --coverage-html-open examples/tests/java/com/pants/examples::
+
+    $ PANTS_TEST_JUNIT_COVERAGE_HTML_OPEN=1 ./pants test examples/tests/java/com/pants/examples::
+
+Pants checks for an environment variable whose name is `PANTS` + the goal name (or goal+task name)
++ the option name; all of these in all-caps, joined by underscores (instead of dots or hyphens).
+
+If an environment variable and a --command-line option both specify a value for some option, the
+command-line option "wins".
+
+For a complete, precedence-ordered list of places Pants looks for option values, see the
+`Options` docstring in
+[src/python/pants/option/options.py](https://github.com/pantsbuild/pants/blob/master/src/python/pants/option/options.py).
+
+Pants checks some environment variables that don't correspond to command-line options.
+E.g., though Pants uses the `PANTS_VERBOSE` environment variable, there's no `--verbose` flag
+to Pants.
+
+* `PANTS_BUILD_ROOT`
+* `PANTS_IVY_CACHE_DIR`
+* `PANTS_IVY_SETTINGS_XML`
+* `PANTS_LEAVE_CHROOT`
+* `PANTS_PY_COVERAGE`
+* `PANTS_PYTHON_TEST_FAILSOFT`
+* `PANTS_VERBOSE`
+
+`pants.ini` Settings File
+-------------------------
+
+Pants can also read command-line options (and other options) from an `.ini` file. For example, if
+your `pants.ini` file contains
+
+    [test.junit]
+    coverage_html_open: True
+
+...then whenever Pants carries out the `test.junit` task, it will behave as if you passed
+`test.junit --coverage-html-open`. If an environment variable and an `.ini` configuration both
+specify a value for some option, the environment variable "wins".
+
+Pants also checks some `.ini` settings that don't correspond to any command-line option.
+For example, though Pants uses the `PANTS_VERBOSE` environment variable, there's no `--verbose`
+flag to Pants.
+
+### Overlay `.ini` Files with `--config-overrides`
+
+Sometimes it's convenient to keep `.ini` settings in more than one file. Perhaps you usually
+operate Pants in one "mode", but occasionally need to use a tweaked set of settings.
+
+Use the `--config-overrides` command-line option to specify a second `.ini` file. Each of
+this `.ini` file's values override the corresponding value in `pants.ini`, if any.
+For example, if your `pants.ini` contains the section
+
+    [test.junit]
+    coverage_html_open: True
+    debug: False
+
+...and you invoke `--config-overrides=quick.ini` and your `quick.ini` says
+
+    [test.junit]
+    coverage_html_open: False
+    skip: True
+
+...then Pants will act as if you specified
+
+    [test.junit]
+    coverage_html_open: False
+    skip: True
+    debug: False