Added missing 'restrict' to some kernels' cntx_t*.

Details:
- Added missing 'restrict' keyword to cntx_t* argument of function
  signatures corresponding to level-1v, level-1f, and level-1m kernels.
  This affected bli_l1v_ker_prot.h, bli_l1f_ker_prot.h, and
  bli_l1m_ker_prot.h. (The 'restrict' was already being used to
  qualify cntx_t* arguments for kernels defined in bli_l3_ker_prot.h.)
- Added comments to bli_l1v_ker.h, bli_l1f_ker.h, bli_l1m_ker.h, and
  bli_l3_ukr.h that help explain how those headers function to produce
  kernel prototypes using the prototype macros defined in the files
  mentioned above.

ref_kernels/1/bli_xpbyv_ref.c

@@ -39,12 +39,12 @@
 \
 void PASTEMAC3(ch,opname,arch,suf) \
      ( \
-       conj_t          conjx, \
-       dim_t           n, \
-       ctype* restrict x, inc_t incx, \
-       ctype* restrict beta, \
-       ctype* restrict y, inc_t incy, \
-       cntx_t*         cntx  \
+       conj_t           conjx, \
+       dim_t            n, \
+       ctype*  restrict x, inc_t incx, \
+       ctype*  restrict beta, \
+       ctype*  restrict y, inc_t incy, \
+       cntx_t* restrict cntx  \
      ) \
 { \
 	ctype* restrict chi1; \