[DOC] ENV keys must be strings [ci skip]

* hash.c (env_aset): state that ENV keys must be strings.

* process.c (rb_f_spawn): ditto.  [ruby-core:68146] [Bug #10859]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@49635 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

hash.c

@@ -3034,6 +3034,7 @@ ruby_unsetenv(const char *name)
  *
  * Sets the environment variable +name+ to +value+.  If the value given is
  * +nil+ the environment variable is deleted.
+ * +name+ must be a string.
  *
  */
 static VALUE

process.c

@@ -3950,6 +3950,8 @@ rb_f_system(int argc, VALUE *argv)
  *    env: hash
  *      name => val : set the environment variable
  *      name => nil : unset the environment variable
+ *
+ *      the keys must be strings.
  *    command...:
  *      commandline                 : command line string which is passed to the standard shell
  *      cmdname, arg1, ...          : command name and one or more arguments (This form does not use the shell. See below for caveats.)