[std] Use of \placeholder for atomic operations meta-variables

[jensmaurer]

Our current (undocumented) guidance is

• \tcode{foo} is used for stuff that may occur in source code, given a suitable environment.
• \placeholder is used for code meta-variables, e.g. exposition-only types and concepts
• Other meta-variables and indexing use math mode: $\tcode{T}_i$

First, we should document this policy. Second, we should also apply it to the atomics meta-variables when constructing all sorts of partial orders throughout [basic] and [atomics]. That is, these should change from \placeholder to $math mode$.

[jensmaurer]

Editorial teleconference: \placeholder is a variable to be replaced by source code. Add examples. Agreed with the changes in [basic] and [atomics] for "atomic operation A" and "memory location M" etc.

[jensmaurer]

Rules documented in wiki.

[jensmaurer]

Uh, using $M$ for the memory model placeholders has an unfortunate side-effect: We lose the italics correction:
Before:

After:

Not good. (Branch c24.)

[jensmaurer]

Editorial teleconference: Wait for more LaTeX expertise to be present.

[jensmaurer]

Editorial meeting: This change is actually intended; math-mode should stand out and not run into the surrounding text. We're fine with the extra space.