Add [0]^\ell to the Notation section.

Signed-off-by: Daira Hopwood <daira@jacaranda.org>

protocol/protocol.tex

@@ -939,6 +939,8 @@ $\ascii{...}$ means the given string represented as a
 sequence of bytes in US-ASCII. For example, $\ascii{abc}$ represents the
 byte sequence $[\hexint{61}, \hexint{62}, \hexint{63}]$.
 
+$\zeros{\ell}$ means the sequence of $\ell$ zero bits.
+
 $a..b$, used as a subscript, means the sequence of values
 with indices $a$ through $b$ inclusive. For example,
 $\AuthPublicNew{\allNew}$ means the sequence $[\AuthPublicNew{\mathrm{1}},
@@ -2054,7 +2056,8 @@ In bit layout diagrams, each box of the diagram represents a sequence of bits.
 Diagrams are read from left-to-right, with lines read from top-to-bottom;
 the breaking of boxes across lines has no significance.
 The bit length is given explicitly in each box, except for the case of a single
-bit, or for the notation $\zeros{n}$ which represents the sequence of $n$ zero bits.
+bit, or for the notation $\zeros{\ell}$ representing the sequence of $\ell$
+zero bits.
 
 The entire diagram represents the sequence of \emph{bytes} formed by first
 concatenating these bit sequences, and then treating each subsequence of 8 bits