get_filename_component: Document path components more clearly (#14091)

Organize component names in a table to explain each in more detail.
Clearly state that PATH is the directory name.

Source/cmGetFilenameComponentCommand.h

@@ -62,15 +62,16 @@ public:
   virtual const char* GetFullDocumentation() const
     {
     return
-      "  get_filename_component(<VAR> FileName\n"
-      "                         PATH|ABSOLUTE|NAME|EXT|NAME_WE|REALPATH\n"
-      "                         [CACHE])\n"
-      "Set <VAR> to be the path (PATH), file name (NAME), file "
-      "extension (EXT), file name without extension (NAME_WE) of FileName, "
-      "the full path (ABSOLUTE), or the full path with all symlinks "
-      "resolved (REALPATH).  "
-      "Note that the path is converted to Unix slashes format and has no "
-      "trailing slashes. The longest file extension is always considered. "
+      "  get_filename_component(<VAR> <FileName> <COMP> [CACHE])\n"
+      "Set <VAR> to a component of <FileName>, where <COMP> is one of:\n"
+      " PATH      = Directory without file name\n"
+      " NAME      = File name without directory\n"
+      " EXT       = File name longest extension (.b.c from d/a.b.c)\n"
+      " NAME_WE   = File name without directory or longest extension\n"
+      " ABSOLUTE  = Full path to file\n"
+      " REALPATH  = Full path to existing file with symlinks resolved\n"
+      "Paths are returned with forward slashes and have no trailing slahes. "
+      "The longest file extension is always considered. "
       "If the optional CACHE argument is specified, the result variable is "
       "added to the cache.\n"
       "  get_filename_component(<VAR> FileName\n"