issue doxygen#10820 Snippet in same file to share docs

When having e.g.:
```
## \file

# [common_param_a]
#  @param a This is a common parameter that appears in many functions.
# [common_param_a]

## @brief This is a function.
#
# @snippetdoc example.py common_param_a
# @param b This is another parameter.
def function1(a, b):
    pass
```
or analogous in Fortran:
```
!> \file

! [ftn_common_param_a]
!  @param a This is a common parameter that appears in many functions.
! [ftn_common_param_a]

!> @brief This is a function.
!>
!> @snippetdoc this ftn_common_param_a
!> @param b This is another parameter.
subroutine ftn_function1(a, b)
    INTEGER a
    INTEGER b
end subroutine
```
we get warnings like:
```
example.py:4: warning: '\param' command is not allowed in section title, ending section title.
```
and in Fortran the `!` appears in the output

Whilst in C++ the analogous example:
```
/// \file

/** @brief This is a function.
 *  @snippet{doc} this cpp2_common_param_a
 *  @param b This is another parameter. */
void cpp2_function1(int a, int b){}

/*  [cpp2_common_param_a]
 *  @param a This is a common parameter that appears in many functions.
 *  [cpp2_common_param_a]
*/
```
works OK, due to the rule
```
<DocBlock>"\\ilinebr "{B}*"*"/[^/]      {
                                          QCString indent;
                                          indent.fill(' ',computeIndent(yytext+8,yyextra->column));
                                          yyextra->docBlock << "\\ilinebr " << indent;
                                        }
```
added analogous rules for python and Fortran.

src/fortranscanner.l

@@ -1521,6 +1521,13 @@ private                                 {
                                           yyextra->blockLineNr=yyextra->lineNr;
                                           BEGIN(DocCopyBlock);
                                         }
+<DocBlock>"\\ilinebr "{B}*"!"           {
+                                          QCString indent;
+                                          indent.fill(' ',yyleng-9);
+                                          yyextra->docBlock += "\\ilinebr ";
+                                          yyextra->docBlock += indent;
+                                        }
+
 <DocBlock>[^@*`~\/\\\n]+                { // any character that isn't special
                                           yyextra->docBlock += yytext;
                                         }

src/pyscanner.l

@@ -1507,6 +1507,12 @@ ID        [a-z_A-Z%]+{IDSYM}*
     ({CMD}{CMD}){ID}/[^a-z_A-Z0-9] { // escaped command
                                      yyextra->docBlock+=yytext;
                                   }
+    "\\ilinebr "{B}*"#" {
+                          QCString indent;
+                          indent.fill(' ',yyleng-9);
+                          yyextra->docBlock += "\\ilinebr ";
+                          yyextra->docBlock += indent;
+                        }
     [^#\\@\n]+          { // any other stuff
                           yyextra->docBlock+=yytext;
                         }