sagemathgh-41191: Warn on meson target generation when doc-build is enabled
    
fix sagemath#41070

Some time ago it was not enabled by default. Not everyone needs building
the docs.

This avoid wasting 5 minutes of developers' time when they upgrade a
version or add/delete a file. (and maybe some hours of figuring what has
changed)

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- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

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URL: sagemath#41191
Reported by: user202729
Reviewer(s): Dima Pasechnik

Author: Release Manager

src/doc/meson.build

@@ -2,6 +2,8 @@ if not get_option('build-docs')
   subdir_done()
 endif
 
+warning('Documentation building enabled, generating targets may be slow. To disable this, pass -Dbuild-docs=false.')
+
 sphinx_check = py_module.find_installation(required: false, modules: ['sphinx'])
 if not sphinx_check.found()
   warning(