BSD sed is angered by doc/Makefile

[grubles]

$ gmake

...

PYTHONPATH=contrib/msggen python3 contrib/msggen/msggen/__main__.py bundle doc/schemas                                                                                                                             
Combining schemas from /home/user/builds/lightning/doc/schemas into /home/user/builds/lightning/contrib/msggen/msggen/schema.json                                                                                  
Created /home/user/builds/lightning/contrib/msggen/msggen/schema.json from 2 files                                                                                                                                 
msggen cln-grpc/proto/node.proto                                                                                                                                                                                   
Documentation version updated to v24.11.1                                                                
sed: 3: "                                                                                                
    :a;N;$!ba;                                                                                           
    s#\ ...": RE error: trailing backslash (\)                                                           
sed: 3: "                                                                                                
    :a;N;$!ba;                                                                                           
    s#\ ...": RE error: trailing backslash (\)                                                           
sed: 3: "                                                                                                
    :a;N;$!ba;                                                                                           
    s#\ ...": RE error: trailing backslash (\)                                                           
sed: 3: "                                                                                                
    :a;N;$!ba;                                                                                           
    s#\ ...": RE error: trailing backslash (\)      
sed: 3: "                                           
    :a;N;$!ba;     
    s#\ ...": RE error: trailing backslash (\)      
sed: 3: "                                           
    :a;N;$!ba;        
    s#\ ...": RE error: trailing backslash (\)
sed: 3: "                                           
    :a;N;$!ba;         
    s#\ ...": RE error: trailing backslash (\)      
sed: 3: "                                           
    :a;N;$!ba;     
    s#\ ...": RE error: trailing backslash (\)      
sed: 3: "                                           
    :a;N;$!ba;           
    s#\ ...": RE error: trailing backslash (\)      
sed: 3: "                                           
    :a;N;$!ba;                                      
    s#\ ...": RE error: trailing backslash (\)      
sed: 3: "                                           
    :a;N;$!ba;                                      
    s#\ ...": RE error: trailing backslash (\)      
tail: stdout: Broken pipe                           
sed: 3: "                                           
    :a;N;$!ba;                                      
    s#\ ...": RE error: trailing backslash (\)      
sed: 3: "                                           
    :a;N;$!ba;  
    s#\ ...": RE error: trailing backslash (\)      
sed: 3: "                                           
    :a;N;$!ba;                                      
    s#\ ...": RE error: trailing backslash (\)      
sed: 3: "                                                       

...

It seems like for portability purposes, we need sed -E?

     -E      Interpret regular expressions as extended (modern) regular
             expressions rather than basic regular expressions (BRE's).  The
             re_format(7) manual page fully describes both formats.

GNU sed also suggests -E:

       -E, -r, --regexp-extended

              use extended regular expressions in the script (for portability use POSIX -E).