doc: clarify the operation of wc -L

* src/wc.c (usage): State that it calculates display width.
* doc/coreutils.texi (wc invocation): Detail the distinct
items used to determine the display width.

doc/coreutils.texi

@@ -3593,7 +3593,10 @@ Print only the newline counts.
 @itemx --max-line-length
 @opindex -L
 @opindex --max-line-length
-Print only the maximum line lengths.
+Print only the maximum display widths.
+Tabs are set at every 8th column.
+Display widths of wide characters are considered.
+Non-printable characters are given 0 width.
 
 @macro filesZeroFromOption{cmd,withTotalOption,subListOutput}
 @item --files0-from=@var{file}

src/wc.c

@@ -134,7 +134,7 @@ the following order: newline, word, character, byte, maximum line length.\n\
       --files0-from=F    read input from the files specified by\n\
                            NUL-terminated names in file F;\n\
                            If F is - then read names from standard input\n\
-  -L, --max-line-length  print the length of the longest line\n\
+  -L, --max-line-length  print the maximum display width\n\
   -w, --words            print the word counts\n\
 "), stdout);
       fputs (HELP_OPTION_DESCRIPTION, stdout);