docs: fix up some comp docs that had rotted a bit

The docs referred to an old name of an internal data structure.  The code
changed in a7a7fd39a but i forgot to update the docs.

The new name includes the underscore character "_", which is meaningful
to asciidoc.  This commit changes the markup delineating the name from
single-tick "'" to backtick "`", to tell asciidoc to pass through the
text without further substitution.  This also changes the rendering
from italic to monospace, which is arguably better anyway (though it's
somewhat inconsistent with the surrounding markup).

This commit also fixes a markup syntax blooper in the FOR_ALL_INSTS()
documentation.
add a very basic test of the biquad component
biquad: fix a crash-when-enabled bug

This fixes a bug in the biquad comp that would cause a crash with a NULL
pointer dereference immediately upon the user setting the .enable HAL
pin to True.  Thanks to forum user gaston48 for the bug report.

The biquad comp exports a hal function called "_" in the .comp and .c
files, called "biquad.0" in hal.  This function calls a function internal
to the biquad component (not exported to hal, not handled by comp(1))
called "Biquad_CalcCoeffs".  The Biquad_CalcCoeffs function accesses the
comp instance's hal pins and params, but this behavior is not supported
by the comp(1) tool.

Only functions handled by the comp tool can access the comp instance's hal
pins and params.  That means functions declared with a .comp "function"
declaration, and the EXTRA_SETUP() function.

This commit moves the Biquad_CalcCoeffs code from its separate function
into the main "_" aka "biquad.0" function, this avoiding the disallowed
"access hal from a called function" problem.
comp: remove an unused and confusing global variable

This commit removes an unused global variable named __comp_state, from
.c files generated by the comp(1) tool.  The global was never set,
so it never served any actual purpose.

The functions declared by "function" declarations in the .comp file,
and the optional EXTRA_SETUP() function, all take an argument which is
also named __comp_state.  Thus, in these functions, the global was masked
by the function argument of the same name.  The __comp_state variable
is used by the convenience defines to access the hal pins and params of
the component instance.  The functions all worked fine and accessed the
hal pins via their argument, just like they should.

The problem was that the global made the convenience defines compile-time
valid everywhere in the .comp, but at run-time any non-"function",
non-EXTRA_SETUP code would access the uninitialized global and crash.

We're better off without it.
Merge remote-tracking branch 'origin/v2.5_branch' into 2.6
fix NULL pointer deref in 7i65 driver