says that rendition and namespace are the only allowed children of tagsDecl, but
shows tagUsage as a third allowed child.
I think the documentation is reasonably clear -- <tagUsage> is listed not as a child of <tagsDecl> but rather as one of the three elements through which "the information is conveyed". The following paragraph clearly states that <tagsDecl> contains "... one or more <namespace> elements, containing a series of <tagUsage> elements, one for each distinct element from that namespace ...".
But that said, clearly if a user glances at this section, the list of three elements leaves the wrong impression. And the fact that a TEI person as experienced and smart as Kevin misconstrues this means it should be more obvious. Perhaps the prose before the list can be updated to make it more obvious where <tagUsage> goes. Or perhaps, since <tagUsage> is *never* used anywhere else except in <namespace>, this can be made explicit in the <desc> element in the <tagUsage> element spec. (And thus that would show up in the list of 3.)
The text doesn't actually say that the three elements listed are all direct children of tagsDecl; it says these elements are used to record three different kinds of information. However, for the benefit of those unaccustomed to close reading, I've now revised the paragraph following the speclist as follows:
<p>The <gi>tagsDecl</gi> element consists of an optional sequence of
<gi>rendition</gi> elements, each of which must bear a unique
identifier, followed by an optional sequence of one or more <gi>namespace</gi>
elements, each of which contains a series of <gi>tagUsage</gi> elements, one
for each distinct element from that namespace occurring within the outermost
<gi>text</gi> element of a TEI document. Note that these
<gi>tagUsage</gi> elements must be nested within a <gi>namespace</gi>
element, and cannot appear directly within the <gi>tagsDecl</gi>