doc: explain why location's "column" are defined vaguely

Suuggested by Frank Heckenbach.
<https://lists.gnu.org/r/bug-bison/2022-01/msg00000.html>

* doc/bison.texi (Location Type): Explain why location's "column" are
defined vaguely.
Show tab handling in ltcalc and calc++.
* examples/c/bistromathic/parse.y: Show tab handling.
* examples/c++/calc++/calc++.test,
* examples/c/bistromathic/bistromathic.test:
Check tab handling.

examples/c++/calc++/calc++.test

@@ -50,6 +50,21 @@ EOF
 run 1 'err: -:2.1: syntax error, unexpected end of file, expecting ( or identifier or number'
 
 
+# Check handling of tabs.
+cat >input <<EOF
+        *1
+EOF
+run 1 'err: -:1.9: syntax error, unexpected *, expecting ( or identifier or number'
+cat >input <<EOF
+	*2
+EOF
+run 1 'err: -:1.9: syntax error, unexpected *, expecting ( or identifier or number'
+cat >input <<EOF
+       	*3
+EOF
+run 1 'err: -:1.9: syntax error, unexpected *, expecting ( or identifier or number'
+
+
 # LAC finds many more tokens.
 cat >input <<EOF
 a := 1

examples/c/bistromathic/bistromathic.test

@@ -366,3 +366,26 @@ err: 1.15: syntax error: expected - or ( or number or function or variable befor
 err:     1 | (1++2) + 3 +  ''
 err:       |               ^
 '
+
+# Check handling of literal tabs.  "Escape" them with a C-v, so that
+# they are not processed as completion requests.
+cat >input<<EOF
+        *1
+	*2
+     	*3
+EOF
+# readline processes the tabs itself, and replaces then with spaces.
+run -n 0 '>         *1
+>       *2
+>       *3
+> ''
+err: 1.9: syntax error: expected end of file or - or ( or exit or number or function etc., before *
+err:     1 |         *1
+err:       |         ^
+err: 2.9: syntax error: expected end of file or - or ( or exit or number or function etc., before *
+err:     2 | 	*2
+err:       |         ^
+err: 3.9: syntax error: expected end of file or - or ( or exit or number or function etc., before *
+err:     3 |      	*3
+err:       |         ^
+'

examples/c/bistromathic/parse.y

@@ -308,14 +308,15 @@ yylex (const char **line, YYSTYPE *yylval, YYLTYPE *yylloc,
 {
   int c;
 
-  // Ignore white space, get first nonwhite character.
+  // Get next character, ignore white spaces.
   do {
     // Move the first position onto the last.
     yylloc->first_line = yylloc->last_line;
     yylloc->first_column = yylloc->last_column;
-
-    yylloc->last_column += 1;
     c = *((*line)++);
+    // Tab characters go to the next column multiple of 8.
+    yylloc->last_column +=
+      c == '\t' ? 8 - ((yylloc->last_column - 1) & 7) : 1;
   } while (c == ' ' || c == '\t');
 
   switch (c)