bug#37212: Incorrect --exclude description with 'grep --help'

[Duncan Moore]

'grep --help' incorrectly states that the --exclude option operates on
directories. Here's the suggested change to the help text:

-      --exclude=GLOB        skip files and directories matching GLOB
+      --exclude=GLOB        skip files matching GLOB

It might also be worthwhile changing 'that match' to 'matching' in these
2 options to give consistent phrasing across the 4 include/exclude
options, to make it a bit easier for the user to compare them:

-      --include=GLOB        search only files that match GLOB (a file
pattern)
+      --include=GLOB        search only files matching GLOB (a file
pattern)

-      --exclude-dir=GLOB    skip directories that match GLOB
+      --exclude-dir=GLOB    skip directories matching GLOB