[Bug target/124140] gcc/doc/riscv-ext.texi does not have an explicite license

[sandra at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124140

sandra at gcc dot gnu.org changed:

           What    |Removed                     |Added
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                 CC|                            |sandra at gcc dot gnu.org

--- Comment #2 from sandra at gcc dot gnu.org ---
Most other .texi files in the doc directory say

@c For copying conditions, see the file gcc.texi

Is that what we want to apply uniformly to all of them?