Improve documentation of 'project-mode-line'

* etc/NEWS:
* doc/emacs/maintaining.texi (Projects):
* lisp/progmodes/project.el (project-mode-line): Improve the
documentation of 'project-mode-line'.  (Bug#66317)
10 jobs for master in 136 minutes and 15 seconds (queued for 7 seconds)
Status Job ID Name Coverage
  Build Images
passed #75291
build-image-inotify

01:34:07

 
  Normal
failed #75292
test-all-inotify

00:42:03

 
  Platform Images
passed #75293
build-image-eglot

01:49:02

failed #75295
build-image-gnustep

00:34:02

passed #75294
build-image-tree-sitter

01:48:54

 
  Platforms
passed #75296
test-eglot

00:02:25

skipped #75298
test-gnustep
passed #75297
test-tree-sitter

00:02:18

 
  Native Comp Images
failed #75299
build-native-comp-speed2

00:07:51

 
  Native Comp
skipped #75300
test-native-comp-speed2
 
Name Stage Failure
failed
test-all-inotify Normal
Uploading artifacts for failed job
Uploading artifacts...
test-all-inotify-d51f3165/: found 531 matching files and directories

Uploading artifacts as "archive" to coordinator... 201 Created
id=75292 responseStatus=201 Created token=ZUDXsiLz
Uploading artifacts...
test-all-inotify-d51f3165/junit-test-report.xml: found 1 matching files and directories

Uploading artifacts as "junit" to coordinator... 201 Created
id=75292 responseStatus=201 Created token=ZUDXsiLz
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 2

failed
build-image-gnustep Platform Images
#12 1410. /usr/include/GNUstep/AppKit/NSDocumentController.h:127:56: warning: this use of "defined" may not be portable [-Wexpansion-to-defined]
#12 1410. 127 | #if OS_API_VERSION(MAC_OS_X_VERSION_10_8, GS_API_LATEST)
#12 1410. | ^
#12 1410. /usr/include/GN
Running after script...
$ test -n "$(docker ps -aq -f name=${test_name})" && docker cp ${test_name}:checkout/test ${test_name}
$ test -n "$(docker ps -aq -f name=${test_name})" && docker rm ${test_name}
find: build-image-gnustep-d51f3165: No such file or directory
$ find ${test_name} ! \( -name "*.log" -o -name ${EMACS_TEST_JUNIT_REPORT} \) -type f -delete
ERROR: Job failed: exit code 1
failed
build-native-comp-speed2 Native Comp Images
Error response from daemon: Get "https://emba.gnu.org:5050/v2/": Get "https://emba.gnu.org//jwt/auth?account=gitlab-ci-token&client_id=docker&offline_token=true&service=container_registry": net/http: request canceled (Client.Timeout exceeded while awaiting headers) (Client.Timeout exceeded while awaiting headers)
Running after_script
Running after script...
$ test -n "$(docker ps -aq -f name=${test_name})" && docker cp ${test_name}:checkout/test ${test_name}
$ test -n "$(docker ps -aq -f name=${test_name})" && docker rm ${test_name}
$ find ${test_name} ! \( -name "*.log" -o -name ${EMACS_TEST_JUNIT_REPORT} \) -type f -delete
find: build-native-comp-speed2-d51f3165: No such file or directory
Cleaning up project directory and file based variables
ERROR: Job failed: exit code 1