Fix bug in std dirs drop; improve documentation (#9449)

# Description

When the directory stack contains only two directories, then std dirs
drop
used to misbehave and (essentially) drop the other directory, instead of
dropping the current directory.

This is fixed by always cd'ing for std dirs drop.

Before:

    /tmp〉enter ..
    /〉dexit
    /〉

After:

    /tmp〉enter ..
    /〉dexit
    /tmp〉


Additionally, I propose to explain the relevant environment variables a
bit
more thoroughly.


# User-Facing Changes

- Fix bug in dexit (std dirs drop) when two directories are remaining

# Tests + Formatting

Added a regression test. Made the existing test easier to understand.

crates/nu-std/tests/test_dirs.nu

@@ -14,7 +14,7 @@ def before-each [] {
 
     mkdir $base_path $path_a $path_b
 
-    {base_path: $base_path, path_a:$path_a, path_b: $path_b}
+    {base_path: $base_path, path_a: $path_a, path_b: $path_b}
 }
 
 def after-each [] {
@@ -46,6 +46,7 @@ def test_dirs_command [] {
     # the def-env gets messed up
     use std dirs
 
+    # Stack: [BASE]
     assert equal [$c.base_path] $env.DIRS_LIST "list is just pwd after initialization"
 
     dirs next
@@ -54,24 +55,33 @@ def test_dirs_command [] {
     dirs prev
     assert equal $c.base_path $env.DIRS_LIST.0 "prev wraps at top of list"
 
+    # Stack becomes: [base PATH_B path_a]
     dirs add $c.path_b $c.path_a
     assert equal $c.path_b $env.PWD "add changes PWD to first added dir"
     assert length $env.DIRS_LIST 3 "add in fact adds to list"
     assert equal $c.path_a $env.DIRS_LIST.2 "add in fact adds to list"
 
+    # Stack becomes: [BASE path_b path_a]
     dirs next 2
     # assert (not) equal requires span.start of first arg < span.end of 2nd
     assert equal $env.PWD $c.base_path "next wraps at end of list"
 
+    # Stack becomes: [base path_b PATH_A]
     dirs prev 1
     assert equal $c.path_a $env.PWD "prev wraps at start of list"
     cur_dir_check $c.path_a "prev wraps to end from start of list"
 
+    # Stack becomes: [base PATH_B]
     dirs drop
     assert length $env.DIRS_LIST 2 "drop removes from list"
     assert equal $env.PWD $c.path_b "drop changes PWD to previous in list (before dropped element)"
 
     assert equal (dirs show) [[active path]; [false $c.base_path] [true $c.path_b]] "show table contains expected information"
+
+    # Stack becomes: [BASE]
+    dirs drop
+    assert length $env.DIRS_LIST 1 "drop removes from list"
+    assert equal $env.PWD $c.base_path "drop changes PWD (regression test for #9449)"
 }
 
 def test_dirs_next [] {