* gdbint.texinfo (FUNCTION_START_OFFSET): Document.

gdb/doc/gdbint.texinfo

@@ -1440,6 +1440,20 @@ function end symbol is 0.  For such targets, you must define
 @code{FUNCTION_EPILOGUE_SIZE} to expand into the standard size of a
 function's epilogue.
 
+@item FUNCTION_START_OFFSET
+An integer, giving the offset in bytes from a function's address (as
+used in the values of symbols, function pointers, etc.), and the
+function's first genuine instruction.
+
+This is zero on almost all machines: the function's address is usually
+the address of its first instruction.  However, on the VAX, for example,
+each function starts with two bytes containing a bitmask indicating
+which registers to save upon entry to the function.  The VAX @code{call}
+instructions check this value, and save the appropriate registers
+automatically.  Thus, since the offset from the function's address to
+its first instruction is two bytes, @code{FUNCTION_START_OFFSET} would
+be 2 on the VAX.
+
 @item GCC_COMPILED_FLAG_SYMBOL
 @item GCC2_COMPILED_FLAG_SYMBOL
 If defined, these are the names of the symbols that GDB will look for to